SIP trunks — keep the PBX, lose the ISDN bill.
Modern call delivery from Gamma, Voiceflex and partner carriers. Plug into your existing PBX, port every number, gain inbound failover and pay sensibly per channel.
ISDN30 is being switched off — and it costs you 3-5x more than SIP.
If your PBX is still happy and depreciated, you don’t need to rip it out. Put SIP trunks behind it, port the numbers across, and you’re modern on the carriage side without an expensive platform swap.
PSTN switch-off ready
Openreach is withdrawing PSTN and ISDN by January 2027. SIP is the standards-based replacement — and gets cheaper every year.
Lower call costs
No more per-channel rental at ISDN30 prices. Pay per concurrent channel, with inclusive UK and most international minute bundles.
Geographic numbers anywhere
Want a London number for the sales line, a Manchester number for the office and an Edinburgh number for the support desk — all answered in one place? SIP makes that trivial.
Inbound resilience
If your PBX goes offline or your circuit drops, calls auto-divert to mobiles, voicemail or an alternative site within seconds. ISDN cannot do that.
Pooled channels
Share channels across multiple offices rather than buying ISDN30s for each. Pay for what you actually use, not what you might use.
Easy add / remove
Need ten more channels for a busy month? Add them in minutes. Drop them again afterwards. Try doing that with ISDN.
Cut the ISDN, keep the phone system, lose half the bill.
Most SIP migrations are quicker, cheaper and less disruptive than businesses expect. The PBX stays — we just change the lines feeding into it. If your PBX is too old to support SIP natively, a small gateway sits in front of it.
- ✓Audit the PBXWe check your existing PBX make, model and firmware. 90% of post-2015 systems support SIP natively. Older systems get a SIP-to-ISDN gateway in front.
- ✓Port the numbersEvery existing number ported across to the SIP carrier — including DDIs, 0800 / 0808 / 03xx, fax lines (where retained).
- ✓Provision the trunksChannels sized to your concurrent call volume plus 20% headroom. Inbound failover routes configured to mobile / alt-site / voicemail.
- ✓Cut over out of hoursSIP goes live, ISDN goes off the next morning. Most users don’t notice the change happened — that’s the win.
What you get with Firstnet SIP trunks
Carriage
- ✓SIP trunks from Gamma Horizon, Voiceflex or matched carrier
- ✓Concurrent channels sized to your peak — pay per channel, not per number
- ✓UK geographic, non-geographic and 0800 / 0808 / 03xx numbers
- ✓Inclusive UK landline and mobile bundles available
- ✓International rates 50-70% below standard PSTN tariffs
- ✓IPv4 + IPv6, TLS encryption, SRTP media
Resilience & service
- ✓Inbound failover to mobile, alt-site or voicemail
- ✓Geo-redundant carrier platforms with 99.999% SLAs
- ✓Bandwidth budgeting and QoS guidance for your edge router
- ✓PBX-side configuration done for you (FreePBX, 3CX, Mitel, Avaya, NEC, Panasonic, etc.)
- ✓Number porting management end-to-end
- ✓UK helpdesk on the same SLA as your IT support
Frequently asked questions
When does PSTN actually switch off?
Openreach has confirmed January 2027 as the full withdrawal date. Stop-sell of new PSTN and ISDN orders is already in place across the UK — you can’t order new ISDN30s. Migration sooner is cheaper and lower-risk than rushing through 2026.
Do we need a new PBX to use SIP?
Not necessarily. Most PBX systems sold in the last 7-10 years support SIP natively. Older systems use a small SIP-to-ISDN gateway that sits in front of the PBX and translates between the two. Either way, the desk phones, training, dial plans and routing stay the same.
What bandwidth do SIP calls use?
Roughly 100 kbps per concurrent call (G.711) or 35 kbps (G.729). A 10-channel SIP trunk needs ~1 Mbps of dedicated headroom. Modern business broadband and leased lines have this comfortably available — we’ll size your circuit and prioritise voice traffic via QoS so calls always get priority over downloads.
Will our existing numbers port?
Yes. Every UK number is portable, including 0800. We coordinate the port window with your existing carrier and schedule it out of hours so the cut-over is brief.
What about fax?
SIP trunks support T.38 fax to a degree, but fax over IP is finicky. Where you absolutely need to keep fax (legal, medical, some local authority workflows) we typically replace the fax line with a fax-to-email service that arrives as a PDF, or retain a single analogue line via an ATA — whichever fits the use case.