Keep your PBX.Replace ISDN with SIP.
SIP trunks deliver voice over your data connection — directly replacing ISDN and analogue PSTN lines while keeping your existing on-premises PBX. Lower call costs, no rip-and-replace, PSTN switch-off ready.
SIP Trunks pain points we hear every week.
If any of these feel familiar, it's not a tech problem — it's a provider problem.
"Our PBX is fine — it's the ISDN bill that hurts."
On-premises PBX from Mitel, Avaya, Panasonic, NEC works perfectly well. But ISDN line rental and call costs are 2-3× what SIP trunks deliver. The PBX isn't the problem — the carrier connection is.
"BT is switching off ISDN in 2027."
All BT ISDN30 and ISDN2 services end January 2027. Many exchanges go dark earlier. If you're still on ISDN, you need a migration plan. SIP trunks are the standard migration path for businesses keeping their PBX.
"We have 30 channels of ISDN30 and pay a fortune."
ISDN30 channels cost £25-40/channel/month plus call charges. SIP trunks: £5-12/channel/month, often with bundled minutes. Even 10 channels of ISDN can save £200-400/month on SIP.
"Our PBX engineer wants £4,000 to upgrade."
PBX vendors love selling new systems. But if your existing PBX is still working, SIP trunks let you keep it for 5-10 more years. The PBX just needs SIP card capability — most modern PBXs already have it.
"We have multiple sites with separate ISDN lines."
Each site has its own ISDN install, separate bill, separate fault management. SIP trunks consolidate to one trunk group, one bill, geo-redundant routing — voice routes intelligently across sites.
"International call costs are absurd."
ISDN international rates are stuck in 2005. SIP trunk pricing for international is typically 50-70% lower. If you have meaningful international call volume, SIP pays for itself quickly.
SIP Trunks done properly.
What you get when Telexico builds it.
Direct ISDN replacement
SIP trunks connect to your existing on-premises PBX via the same SIP protocol the PBX already speaks. No new handsets, no new PBX, no staff retraining. The trunk is the only thing that changes.
30-50% lower call costs
Typical UK SMB saves £300-1,500/month switching from ISDN to SIP. The savings come from line rental, channel rental, and call charges all dropping. Bundled minutes available.
Number porting included
All your existing business numbers port from BT/Virgin/whoever onto the SIP trunk. Same numbers, same dialing, customer-invisible migration. 5-10 working days typical.
Failover routing
Primary SIP fails? Calls auto-divert to mobile, alternate site, or AI receptionist. ISDN can't do this. SIP can do it intelligently.
Geographic flexibility
SIP trunks are software-defined — your business numbers can ring anywhere with internet. Run UK numbers in a Spanish office, route to UK staff working from home, scale instantly with growth.
Full PBX support
Tested compatibility with major UK PBX brands: Mitel, Avaya, Cisco, Panasonic, NEC, 3CX, FreePBX, Asterisk. We handle the SIP configuration and integration with your PBX engineer.
Built for businesses that need reliability, speed and control.
We work with the PBX you already have.
SIP trunks are a carrier-side change, not a phone-system change. Your existing on-premises PBX stays exactly as it is — only the line going into it changes from ISDN/PSTN to SIP. We've integrated SIP trunks with most major UK PBX brands. SIP trunk quality depends entirely on the underlying <a href="/business-internet.html">business internet</a> — particularly upload symmetry, jitter and packet loss — which is why Telexico configures the <a href="/business-connectivity.html">connectivity</a> and the SIP trunks as one integrated install rather than separate purchases.
ISDN → SIP without operational risk.
SIP trunk migration is well-rehearsed. We've done hundreds of UK businesses through it over the last decade. The pattern that works:
PBX capability audit
We confirm your PBX supports SIP — what version, what SIP card if needed, what license count, what concurrent-call capacity. Many older PBXs need a small upgrade (£200-500); many modern ones are already SIP-ready.
Broadband quality verification
SIP rides on your internet connection. We test current broadband: bandwidth, jitter, packet loss, latency variation. If broadband isn't SIP-grade, we recommend a fix BEFORE deploying SIP — usually a broadband upgrade or QoS-aware router.
SIP trunk provisioning
Telexico SIP trunk service provisioned with the right channel count, bundled minutes if applicable, number porting requested for your existing business numbers. Provisioned but not yet active.
Parallel-running test
SIP trunk activated on test extensions while ISDN remains primary. Outbound calls tested via SIP, inbound calls still on ISDN. Voice quality measured. Issues identified before any customer-visible cutover.
Number porting cutover
Numbers port from old ISDN provider to Telexico SIP trunk. Out-of-hours scheduled — typically Saturday morning. Customer-visible disruption: a few minutes.
ISDN decommissioning
After 30-60 days of confirmed stable SIP operation, ISDN lines are formally cancelled with old provider. Channel rental stops. The savings on the next bill are usually the easiest sale we ever make.
Review my current setup.
Not ready to switch yet? Send us your current contracts, bills, or photos of your existing equipment. We'll review what you have, what you're paying, and where you could simplify, consolidate or improve — without any pressure to buy anything from us.
We benchmark your existing broadband, phones, mobile and IT against current UK market pricing and what your business actually needs.
Real engineer review of your current connectivity, voice setup, WiFi, security and continuity — strengths, gaps, and where you're overpaying or underprotected.
We tell you honestly what's available at your postcode — FTTP, leased line, alt-net carriers — and which makes commercial sense for your operation.
For businesses still on ISDN or aging on-premises PBX — an honest cost-and-feature comparison before the 2027 BT switch-off forces a rushed decision.
No hard sell. No fixed package pressure. If we're not a better fit, we'll tell you straight — and recommend what is.
What businesses gain by switching to Telexico.
How we differ from the big providers — written plainly, no sales spin.
Send us your current renewal. We'll review what you have, what you pay, and where we can simplify — no fixed-package pressure, no obligation.
What you actually get from Telexico.
Honest about scope. No aggressive sales tactics, no surprise renewal jumps, no tier-1 call-centre triage. Real UK engineers, transparent pricing, one provider relationship across the stack.
UK-based provider
Wolverhampton-headquartered. Engineers cover the West Midlands daily; UK-wide install via our partner network. Real UK engineer support, UK data residency, UK contractual relationship — not US-routed SaaS.
Real engineer support
When you call Telexico, you reach someone who can actually fix things. Response SLA backed by real engineering capacity rather than call-centre headcount. Named account manager for ongoing customers.
Free infrastructure review
Every engagement starts with a no-obligation audit of your current setup. Honest recommendation — sometimes that's "stay with your current provider after negotiation." We'd rather be honest than oversell.
Transparent pricing
What you sign for is what you pay — including renewal. No teaser pricing that jumps 30-100% at year two. No mid-contract CPI shock. Predictable multi-year cost from day one.
One provider, one platform
Broadband, hosted VoIP, business WiFi, AI Receptionist, 4G/5G failover, CCTV consolidated onto one Telexico relationship. Single bill, single support number, single engineer when something needs attention.
Migration project-managed
Switching to Telexico isn't DIY. We handle contract audit, notice timing, ordering, parallel running, cutover, old-provider close-out. Customer-visible disruption typically measured in minutes.
Simpler telecoms. Fewer suppliers. Real support.
Send us your current setup. We'll show you where we can simplify, consolidate or reduce cost — no rigid pricing, no hard sell.
AI receptionist answers 24/7, captures the lead and sends it straight to your team.
Broadband, phones, AI, WiFi, failover and CCTV — one bill, one UK team.
We design around your team, your hours and your customers — not a generic package.
⚡ 30 seconds to share your details · 💬 We respond promptly · 🔒 No hard sell
SIP Trunks around how you actually operate.
Send us your current setup — we'll review what you have today, what's working, what isn't, and tailor SIP Trunks around your operation. No hard sell, no fixed-package pressure.
Frequently asked questions
What's the difference between SIP trunks and hosted VoIP?
Hosted VoIP replaces your entire phone system with a cloud-based one (no PBX hardware on your premises). SIP trunks replace ONLY the carrier connection — your existing PBX stays, you just swap ISDN/PSTN lines for SIP. Hosted VoIP is more modern and feature-rich; SIP trunks are the right path when you've recently invested in a good PBX and want to preserve it.
Does my PBX support SIP?
Most modern PBXs (post-2010) support SIP trunks natively. Older PBXs may need a SIP card added (£200-500 hardware cost). Very old PBXs (pre-2005) may not be SIP-capable — at that point hosted VoIP is usually the better path. We audit your PBX in the initial survey.
How many channels do I need?
One channel = one concurrent call. If you have 20 staff but rarely have more than 10 on the phone simultaneously, 10 SIP channels is enough. We measure your actual concurrent usage from CDRs before recommending — it's almost always lower than people guess.
What about emergency calls (999)?
Full 999 support is mandatory on SIP trunks just like ISDN. Address registration is required (Ofcom mandate) so emergency services see the right location when dispatched. We handle this during setup.
Is SIP quality as good as ISDN?
On a properly-engineered business broadband (FTTP with QoS): yes, often better than ISDN. On a residential-grade connection or one without QoS: probably worse. SIP needs the underlying internet to be solid. We always verify broadband quality before quoting SIP.
What happens if my internet drops?
Without failover: SIP trunks go down. Standard mitigation: 4G/5G failover keeps SIP trunks live during broadband outages. Or: SIP calls auto-divert to mobile numbers / alternate sites. ISDN doesn't depend on broadband but SIP does — failover planning is essential.
Can I migrate gradually from ISDN to SIP?
Yes — common for larger businesses. Run SIP alongside existing ISDN for 30-90 days, port numbers in batches, decommission ISDN channels as confidence builds. Both can coexist for as long as needed.
How much do SIP trunks cost?
Per-channel pricing: £5-12/month depending on bundle. 10-channel SIP trunk with 5,000 UK minutes included: £80-120/month all-in. Compare to typical 10-channel ISDN30 at £400-600/month and the savings are immediate.