Quick Answer: SIP trunks replace ISDN and analogue lines. Wholesale SIP trunk costs £4-£10/month per trunk + £0.004-£0.012/minute calls. Resellers add value through SBC configuration (Session Border Controllers), connectivity bundling (broadband, leased lines), and PBX migration support. Typical SIP trunk resale price is £15-£25/month per trunk + marked-up call minutes. The PSTN switch-off deadline (31 January 2027) creates urgent demand for SIP migration, especially from legacy devices (alarms, lifts, EPOS terminals).
SIP Trunk Sizing: How Many Trunks Does a Customer Need?
SIP trunk count is determined by concurrent call capacity needed, not total users. This table helps resellers right-size capacity and avoid under/over-provisioning.
| Customer Type / User Count | Typical Concurrent Calls Needed | SIP Trunks Required (Erlang calculation) | Peak Capacity (including headroom) | Monthly Cost at £18/trunk | Annual Cost | Reseller Opportunity (bundled with connectivity) |
|---|---|---|---|---|---|---|
| Small office (10-25 users) | 2-4 concurrent calls | 2-3 trunks | 4 trunks (headroom for growth) | £72/month | £864 | Bundle with 24Mb+ broadband. Lock in £15/month SIP price. Margin: 25-35%. |
| Mid-market SME (50-150 users) | 6-12 concurrent calls | 6-10 trunks | 10-12 trunks (buffer for spike calls) | £180-£216/month | £2,160-£2,592 | Upsell SBC hosting (£300/month), call recording (£50/month). Margin: 30-40%. |
| Enterprise (200-500 users) | 15-30 concurrent calls | 15-25 trunks | 25-30 trunks (redundancy to partner trunk group) | £450-£540/month | £5,400-£6,480 | Cloud SBC hosting, traffic engineering, failover setup (£800/month). Margin: 35-45%. |
| Large enterprise / Multi-site (500+ users) | 40+ concurrent calls | 40-60 trunks | 60+ trunks across 2-3 providers for resilience | £1,080+ per provider/month | £12,960+ | Dedicated account team, custom SLA, geo-redundant SBCs (£2,000+/month). Margin: 40-50%. |
| Contact centre (100-300 concurrent calls needed) | 100-300 concurrent calls | 100-300 trunks (high capacity) | 300+ trunks (multiple providers required for redundancy) | £5,400+ per month | £64,800+ | Wholesale SIP routing optimisation, least-cost routing, international gateway integration. Margin: 25-35% (high volume discounting). |
Codec and Bandwidth Comparison
Codec choice impacts bandwidth, call quality, and licensing costs. This table guides reseller positioning on network requirements and costs.
| Codec | Bandwidth per Call (Mbps) | MOS Score (call quality) | Licence Cost / Patent Fees | Common Use Case | Reseller Recommendation |
|---|---|---|---|---|---|
| G.711 (μ-law, A-law) | 0.064 Mbps (64 kbps) | 4.1 (excellent) | None. Royalty-free. | Enterprise standard. PSTN interop. | Default codec. No negotiation needed. Ideal for ISDN/analogue migration. |
| G.729 | 0.008 Mbps (8 kbps) | 3.92 (good) | Yes. Per-channel licensing (£0.50-£2.00 per channel licence/year). Check vendor patent pool. | Bandwidth-constrained environments. International calls (reduces costs). | Upsell to customers on slow broadband (<5Mb) or with high call volumes (contact centres). Margin increase: 20-30%. |
| G.726 | 0.016-0.040 Mbps (16-40 kbps) | 3.85 (good) | None commonly enforced. | Legacy PBX interop, some carriers. | Rarely recommended. Use G.711 or G.729 instead. |
| iLBC (Internet Low Bitrate Codec) | 0.013-0.015 Mbps (13-15 kbps) | 3.7 (good) | None. Open source (RFC 3951). | Low-bandwidth, high-packet-loss networks (satellite, 3G). | Niche use (remote/field workers on mobile). Not mainstream. |
| Opus | 0.008-0.128 Mbps (8-128 kbps variable) | 4.0+ (excellent to good depending on bitrate) | None. Royalty-free (RFC 6716). | Next-generation WebRTC and cloud voice. Adaptive bitrate. | Position for future-ready customers (Teams Direct Routing, Avaya Cloud, Five9). Mention superior quality on poor networks. |
SBC (Session Border Controller) Feature Comparison for Resellers
SBCs sit between customer PBX and SIP trunk provider. Choose based on customer infrastructure maturity and your hosting model.
| SBC Type / Vendor | Deployment Model | Cost to Reseller (hardware or SaaS) | Key Features | Best Fit Customer | Reseller Value-Add Opportunity |
|---|---|---|---|---|---|
| On-Premise Hardware SBC (Cisco ASA, Avaya Aura SBC) | Customer owns and manages hardware on-site. | £5,000-£30,000 capex (one-time). £2,000-£5,000/year support. | Full protocol conversion, security, NAT traversal, call recording integration. Maximum control. | Enterprise with in-house IT team. Security-sensitive. Legacy PBX deployment. | Configuration services (£3,000-£5,000), annual support retainers (£2,000-£3,000). Margin 60-75%. |
| Hosted Cloud SBC (AWS, Azure, Ribbon, Acme) | Reseller hosts SBC in AWS/Azure. Managed service model. | £200-£500/month (software or infrastructure rental). Shared across customers. | Auto-scaling, geo-redundancy, managed failover, integrated monitoring. Pay-as-you-go. | SME to mid-market. No IT infrastructure. Wants resilience without capex. | Recurring SBC hosting service (£300-£500/month), additional margin 50-65%. Bundled with SIP trunk. |
| Virtual SBC (VoIP.ms, Zoiper, Grandstream Wave) | Software-based. Installed on customer VM or cloud instance. | £0-£200/month software cost. | Basic protocol conversion, limited advanced features. Lightweight. | Cost-sensitive SME. Simple SIP trunk deployment. No legacy PBX features needed. | Low-margin commodity. Use to acquire customer and upsell SIP minutes, call recording. Margin 10-20% on SBC alone. |
| PBX Vendor SBC (Avaya, Cisco, Asterisk embedded) | Built into customer PBX or Asterisk deployment. | Included in PBX licence. £500-£2,000 configuration. | Tight PBX integration, minimal external dependencies. | Customer with existing Avaya or Cisco investment. Minimal external dependencies desired. | Configuration and integration services only (£1,500-£3,000). Focus revenue on SIP trunks and support. |
PSTN Switch-Off Migration Checklist: Device-by-Device Paths
31 January 2027 is the Ofcom and Openreach deadline. Legacy analogue, ISDN, and device-dependent services must migrate to SIP or cloud voice. This table categorises equipment and migration pathways.
| Device / Service Type | PSTN Dependency | Current Status | Recommended Migration Path | Timeline (for Jan 2027 deadline) | Reseller Implementation Complexity | Reseller Revenue Opportunity |
|---|---|---|---|---|---|---|
| Desk phones (Cisco, Avaya, Polycom) | High. Receive inbound calls, dial outbound. | Works on ISDN/analogue. Not IP-enabled. | Replace with SIP phones or cloud client (Teams, Zoom). OR provision SIP firmware upgrade if available. | Audit now (Q1 2026). Replace by Q3 2026. | Low. Phone replacement and configuration is straightforward. | Phone hardware sale (£150-£400 per phone, £3-8% margin). Call SIP trunk (higher margin). |
| PABX / PBX system (Avaya, Siemens, NEC) | Very high. Central switching and routing. | Analogue or ISDN-based. Older models lack IP/SIP capability. | Option A: Replace with cloud voice (Teams Phone, Avaya Cloud). Option B: Upgrade PBX to SIP-capable version. Option C: Add SBC + SIP trunk to existing PBX (hybrid). | Decision: now (Q1). Implementation: Q2-Q3 2026. | Medium-High. Requires PBX assessment, vendor engagement, phased migration. | Large project (£5,000-£30,000 depending on option). Margin 20-40%. |
| Security alarm systems (line seizure calls to monitoring centre) | Very high. Alarm dial-out requires PSTN connection. No IP fallback. | Standalone analogue line. No integration with PBX. | Option A: Install PSTN/SIP gateway (e.g., Grandstream GWN, EDPACS AlarmVoIP) to convert to SIP. Option B: Replace with cloud alarm service (ARC with VoIP). Option C: Dual SIM mobile backup (if allowed by insurer). | Audit alarm systems now. Implement by Q4 2026 (90 days before deadline). | Medium. Requires site survey, gateway selection, monitoring centre certification. | Gateway hardware (£200-£800) + installation (£500-£1,500). Ongoing SIP trunk for alarm outbound (£8/month). Margin 30-40%. |
| Lift emergency phones (dual-line analogue for compliance) | Very high. Legal requirement. Must be able to reach emergency services and building owner. | Typically old analogue lines, often dedicated. | Option A: Install SIP gateway in lift machine room + SIP trunk. Option B: VoIP lift phone module (newer lifts have IP capability). Option C: GSM backup if SIP not available. | Audit lifts in Q1 2026. Implement by Q4 2026. | High. Requires lift engineering certification, coordination with lift maintainer, redundancy validation. | SIP gateway (£400-£1,200) + installation (£800-£2,000). Annual SIP trunk + monitoring (£150-£300). Margin 25-35%. |
| EPOS systems (card payment terminals, restaurant tills) | Medium-High. EPOS may dial out for credit card authorisation or payments. Some modern EPOS over IP. | Older EPOS systems hardwired to analogue lines or mobile modem. | Option A: PSTN/SIP gateway for older EPOS. Option B: Upgrade EPOS to modern IP-capable system (requires till supplier support). Option C: Move payments to IP-only method (card reader over broadband). Option D: Mobile backup (4G modem). | Audit EPOS systems by Q2 2026. Implement by Q3 2026 (before peak trading season). | High. Requires EPOS supplier engagement, payment processor changes, till reconfiguration. | SIP gateway (£300-£600), installation (£500-£1,000). Hardware refresh sales (if upgrading EPOS). Margin 20-30%. |
| Fax machines | Medium. Fax requires PSTN or dedicated modem. | Standalone analogue fax machine. | Option A: Deploy T.38 SIP gateway (e.g., Grandstream GWN). Option B: Cloud fax service (eFax, RingCentral Fax, Avaya Fax). | Audit by Q1. Implement by Q2 2026 (low urgency; most businesses reducing fax use). | Low. Gateway is simple. Cloud fax requires minimal integration. | SIP gateway (£200-£400) or cloud fax subscription (£10-£20/month). Margin 30-50%. |
| Odd/spare lines (backup, guest lines, modem pools) | Medium. Backup lines, receptionist lines, legacy modem lines. | Analogue or ISDN. Often forgotten in audits. | Decommission or convert to SIP. Often resellers find these during PSTN audit and add them to scope. | Audit by Q1 2026. Decommission by Q3 2026. | Low. Often requires just SIP trunk provisioning. | Additional SIP trunks (£15-£25/month per line). Small but 100% margin recovery on trunks. |
Frequently Asked Questions
What is the difference between SIP trunk and ISDN?
ISDN (Integrated Services Digital Network) is circuit-switched technology. SIP trunks are packet-switched VoIP. ISDN is being switched off 31 January 2027. SIP trunks are cheaper, more flexible (can add calls instantly without ordering physical lines), and support modern features (call recording, call routing rules). ISDN provides only voice; SIP trunks support video and data.
Can I migrate a customer from ISDN to SIP trunk without downtime?
Yes. Deploy SBC and SIP trunk first in parallel with ISDN. Test calling, call routing, and emergency services. Once validated, switch inbound calls to SIP trunk. Keep ISDN active for 2-4 weeks as a rollback. Then decommission ISDN. Total migration window: 2-4 weeks with zero downtime if planned correctly.
How many SIP trunks should a customer with 50 ISDN lines purchase?
Not 50 trunks. ISDN 30/2 (30 calling channels) requires roughly 15-20 concurrent calls. Size for 15-20 SIP trunks. ISDN forces customers to pay for all 30 channels whether used or not. SIP is capacity-based: you pay only for concurrent calls. Savings: typically 50-70% on trunk costs.
Does my SIP trunk provider support 999/112 emergency calls?
Most do. Verify in the SLA. SIP trunk provider must pass your emergency calls to the appropriate emergency services (Police, Fire, Ambulance) with accurate caller location data. This is an Ofcom requirement. Do not deploy SIP without confirming emergency call compliance with your provider.
What is caller location and why does it matter for SIP trunks?
When a customer dials 999 from a SIP phone, emergency services need to know the location of the caller. For fixed sites (office), you register the address once. For mobile/nomadic users, location is harder. With SIP, you must document location data and ensure your SIP trunk provider submits it to emergency services. Ofcom enforces this. Resellers must audit and document customer locations.
Can I host an SBC for multiple customers in my data centre?
Yes. Multi-tenant SBC hosting is common and profitable. Customers get resilience (geo-redundant SBCs) without capex. You monetise through hosting fees (£200-£500/month per customer) with 50-70% margin. Requires IP expertise and SLA commitment (99.5% uptime minimum).
What QoS settings should I configure for SIP trunks?
Prioritise SIP traffic (voice) over data. Set DSCP EF (Expedited Forwarding) for voice packets. Target: <150ms latency, <20ms jitter, <1% packet loss. Many broadband connections (residential-grade) cannot meet these. Upsell business-grade connectivity (leased line, FTTP) when customer’s broadband is <20Mb or contended.
Sources
IETF SIP and VoIP Standards (RFC 3261, RFC 3550)
Cisco SBC and SIP Trunk Solutions
Ofcom PSTN Switch-Off and SIP Trunking Requirements
Openreach PSTN Switch-Off Timeline and SIP Migration
ISP Review PSTN Switch-Off and SIP Trunking News
Partner Playbook: SIP Trunk Sales and Migration Cycle
| Phase | Timeline | Key Activities | Deliverables | Reseller Revenue Opportunity |
|---|---|---|---|---|
| PSTN Audit | 2-3 weeks | Site survey. Document all PSTN lines: desk phones, PBX, alarms, lifts, EPOS, fax, odd lines. Identify SIP-ready equipment. | PSTN audit report. Migration recommendations. Equipment inventory. | Audit service: £500-£1,500 per site. Margin 80%. |
| Design Phase | 1-2 weeks | Right-size SIP trunks. Select SBC (on-premise vs cloud). Plan codec, redundancy, failover. Estimate costs. | Migration plan. SIP trunk sizing calculation. Cost estimate (capex + opex 3-year). | Design services: £1,000-£2,500. Margin 60-70%. |
| Hardware Procurement (if applicable) | 1-2 weeks | Order SIP gateway devices, phones, or SBC hardware. Coordinate with suppliers. | Hardware quotes. Delivery schedule. | Hardware markup: 15-25%. Margin varies by device. |
| Parallel Testing and Rollout | 2-3 weeks | Deploy SBC and SIP trunks in parallel with ISDN. Test calling, emergency services, failover. Train staff. | Test results. Staff training certification. SBC configuration documentation. | Implementation services: £2,000-£5,000. Margin 50-65%. |
| Cutover and Decommissioning | 1-2 weeks | Switch inbound calls to SIP. Decommission ISDN after validation period (2-4 weeks). | Cutover sign-off. ISDN cancellation notice to Openreach/BT. | Cutover support: £500-£1,000. Margin 70%. |
| Ongoing Support and Optimisation | Ongoing | Monitor QoS. Optimise codec and routing. Support customer calls and escalations. | Monthly monitoring reports. Quarterly QBRs. | SIP trunk (ongoing): £15-£25/month per trunk. Margin 30-40%. Support retainer: £300-£600/month. Margin 60%. |
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