Quick answer: "Voice retirement" is not one wave, it is two running side by side. On the fixed-line side, regulators and incumbents are retiring the switched copper network, which our PSTN switch-off explainer covers. On the mobile side, operators are shutting their 2G and 3G radios and moving every voice call onto VoLTE (voice over the 4G packet core) or VoNR (voice over the 5G standalone core) — and the IMS layer that carried both is now expensive enough that operators have begun trimming it down to a few essential functions. The umbrella underneath both tracks is IMS: an IP-based call-control core that the industry spent the last two decades deploying, and is now retiring in whole or in part. The tenant-owned response is the same for either track: move your fixed-line voice onto SIP or programmable voice before the medium underneath it is cut, and line up your caller-identity record (STIR/SHAKEN posture, BYO carrier, ported numbers) before the migration window closes.
This is an industry-news explainer — the mobile-side counterpart to the PSTN switch-off and EU migration checklist posts, which cover the fixed-line track. It aims at the same reader (a CPaaS/UCaaS buyer planning a migration) but answers a different gap in the sunset vocabulary. Nothing described here is an Orbit product change; the hooks at the end point at shipped tenant-owned controls, never a roadmap.
The sunset track: 2G/3G → VoLTE → VoNR
The mobile voice story runs in four steps, and each one is a retirement:
- 2G and 3G radios go dark. Operators have been switching off their legacy circuit-switched radios on published national and per-market timelines. The radio bands get repurposed for LTE and 5G; the voice path they used to carry either moves to VoLTE/VoNR or dies.
- SIMs without a VoLTE profile stop placing calls. Once the legacy bearer is gone, the operator's device programme trims to VoLTE-capable handsets. Handsets stuck on 3G-only voice fall off the network at cut-over, which is why the consumer operators publish device whitelists and why the tenant-owned enterprise side needs an inventory.
- Voice moves onto the packet core. "VoLTE" is voice carried as data over an LTE bearer; "VoNR" is voice carried as data over a 5G standalone bearer. Both use IMS (the IP Multimedia Subsystem) as the call-control core, folded into the operator's packet core rather than a circuit-switched one.
- The IMS layer itself is being trimmed. IMS was built when SIP-based call control needed a full separate telephony core — MRFs, session border functions, CSCFs, ENUM/DNS trees. As traffic moved to integer voice rates and simpler SIP interconnects, the operators who built IMS at telco scale have begun retiring the components that bought them nothing: 5G standalone cores run VoNR without the older IMS scaffolding, and MVNOs lease IMS functions their host exposes. "IMS retirement" is the industry's umbrella for that reverse gear.
The regulatory backbone for this wave is national, not global. Each market's telecom regulator blesses the operator's radio-shutdown plan (Ofcom in the UK, the FCC in the US, BNetzA in Germany, the national authority elsewhere), and the industry direction is that the mobile and fixed-line programmes proceed on parallel but separate calendars. The difference for a buyer is one of medium, not posture: where copper dies a physical death on the PSTN side, the mobile sunset cuts a radio bearer — but either way, the voice estate has to move to IP before the medium underneath it is cut.
Why it matters to a PSTN-switch-off thread running in parallel
The two waves share the root cause (the old circuit-switched core is too expensive to keep) and the tenant response (move voice to IP). They diverge on the asset at risk:
- Mobile-side risk is the in-car SIM estate. M2M and IoT SIMs on 2G/3G-only profiles — alarm panels, telemetry boxes, fleet trackers — stop placing voice-bearer commands once the radio is gone. The tenant-owned inventory on the PSTN side (alarm circuits, lift phones, payment terminals) has a mobile-side sibling that the same checklist must cover: every voice-capable mobile bearer your estate depends on.
- The IMS layer decides which of your carrier relationships still terminate. Operators that retire their IMS functions turn down the SIP peering points some wholesale and BYO-carrier relationships ride on. A trunk that terminates onto a retiring IMS edge needs a new peer — a carrier re-peering decision your provider makes, or a port onto the platform's carrier of record.
- Caller identity is a shared two-sided problem. Mobile operators retiring legacy switching still sign and verify the attestation metadata the recipient's device displays. The STIR/SHAKEN record the US wave runs on is the same one your fixed-line SIP and the platform's own wholesale termination carry — the attestation explainer covers what each level asserts and where your tenant-owned policy pins it.
The migration clock is also a shared one. The EU and UK regulators publish the PSTN timeline beside the mobile one because the assets overlap: a telecom estate that runs its alarm panels on 3G SIMs and its PBX trunks on ISDN is one wave with two media. Reading them as one programme is the accurate plan; treating them as separate projects is how the expensive option (the operator's managed conversion) becomes the default by silence.
The per-tenant action: the four controls the platform ships today
The response below uses shipped surfaces, not roadmap items. The tenant (you) owns the migration sequencing and the inventory; the platform supplies the shipping controls that carry it:
- STIR/SHAKEN attestation posture. On Orbit your outbound calls attest at a level your tenant-owned policy defines (
target_attestation,downgrade_handling,inbound_min_verification,flag_unverified_inbound), and the posture snapshot shows which of your originating numbers meet the target. Owning your numbers through the platform attests at full (A); pool or delegate-certificate numbers cap at partial (B). The attestation explainer walks the levels, the fields, and the posture read. - Programmable voice over SIP. The replacement for a retiring circuit-switched path is a SIP bearer your carrier peers with, or the platform's own provisioning and call-control path. The programmable voice guide covers the shipped setup: ingest your carrier's SIP trunk or provision numbers on the platform, then wire the call-control logic — IVR, AI agents, routing — on top. Either way the same outbound rule holds: PSTN egress runs over Devotel's wholesale softswitch.
- BYO carrier and number porting. Where the migration keeps the existing carrier relationship, the platform ingests the carrier's SIP trunk on digest authentication or IP allowlist; where it moves the numbers, the porting flow runs through bulk screening, in-platform LOA signing, and per-stage webhooks. The PSTN checklist lays out the full per-line decision routine.
- Inventory with the fixed-line checklist, extended to mobile SIMs. The PSTN side's tenant-owned inventory (alarms, lifts, fax, payment terminals) needs the mobile sibling (voice-capable SIMs, telemetry boxes, trackers) appended to the same register. A migration plan that inventories one medium and not the other is where the post-cut-over outage lives.
Reader beware, in the plain sense: the published operator timeline for your market is the upper bound, and whatever your carrier tells you in writing is the budget line. The platform's shipped controls are the execution vehicle; the date the land underneath your existing estate disappears is the date you planned against.
Frequently asked questions
What is VoLTE and how is it different from VoNR?
VoLTE carries voice as data over the 4G/LTE packet core; VoNR carries voice as data over the 5G standalone packet core. Both use IMS as the call-control core. The retirement context is the same one: the circuit-switched radio underneath them (2G or 3G) is cut, so every voice call moves onto whichever packet-core bearer the handset supports.
What does "IMS retirement" mean?
Operators built IMS when IP-based call control needed a full telephony core. As traffic moved to simpler SIP interconnects, the operators have begun retiring the components that bought them nothing — "retirement" is the industry's umbrella for trimming IMS back to the essential functions. For a buyer it decides which of their carrier's SIP peering points still terminate, which is a carrier-re-peering decision the migration plan prices.
Does this change the PSTN switch-off plan?
No — it is the parallel wave. The fixed-line copper retirement is one track; the mobile 2G/3G → VoLTE/VoNR transition is the other. Both demand the same tenant-owned response (move voice to IP before the medium is cut) and the same inventory (every voice-capable bearer, fixed-line and mobile).
What tenant-owned controls does Orbit ship for this?
Four shipped surfaces: STIR/SHAKEN attestation posture (target_attestation, downgrade_handling, inbound_min_verification, flag_unverified_inbound), programmable voice over SIP (guide), BYO carrier and number porting, and a shared inventory checklist that treats fixed and mobile bearers as one estate. Outbound PSTN egress runs over Devotel's wholesale softswitch in every posture.
Is the platform retiring anything itself?
No — the explainer is external industry movement, not an Orbit product change. The hooks above point at shipped tenant-owned controls; nothing here promises future work.
The takeaway
"IMS retirement" is the umbrella the industry publishes under when it turns the old telephony core around: 2G/3G radios cut, voice moved onto VoLTE/VoNR, and the IMS functions trimmed back to what the packet core still needs. The fixed-line PSTN switch-off we covered before is the parallel wave with the other medium; both run on national regulator timelines and both demand the same tenant-owned inventory and migration. The news is external. The four controls the platform ships — attestation posture, programmable voice, BYO/porting, one shared inventory — are the execution vehicle you audit against, in your migration plan's own vocabulary.
Published 23 September 2026.