A forwarded call begins as one inbound session to the original DID (or extension) and is then sent to a new target. The original called identity may stay in SIP History-Info or Diversion headers so the far end can see that the call was diverted. On the PSTN, the same idea is Q.732 / ISDN call diversion. On SIP, an endpoint or proxy often answers with 302 Moved Temporarily, or the application server places a new INVITE toward the forward-to number.
CFU, CFB, and CFNR
ITU-T / ISDN supplementary services name three classic conditions. Cloud PBXs keep the same logic even when the UI says "always," "when busy," and "when unanswered."
| Service | Full name | When it fires | Typical SIP mapping |
|---|---|---|---|
| CFU | Call Forwarding Unconditional | Every inbound call, before the original target rings (or immediately after answer at the platform) | 302 Moved Temporarily, or an immediate new INVITE to the forward-to URI |
| CFB | Call Forwarding Busy | The target is busy (all appearances in use, DND treated as busy, or 486 Busy Here) | Busy (486 / 600) then divert. History-Info cause 486 is common. |
| CFNR | Call Forwarding No Reply (no answer) | The target rings until a timer expires and nobody answers | After alerting: 408 / 480 style no-answer, then divert. CFNA is the same idea. |
SIP 302 and how forwarding is signaled
RFC 3261 defines 302 Moved Temporarily: the previous hop should retry the URI in Contact. That is a clean CFU when a proxy or UA refuses to ring locally. In practice, hosted PBXs often hairpin: they accept the inbound INVITE, then originate a second leg to the cell or desk. Hairpinning uses two call paths (an inbound termination leg plus an outbound leg) and can change caller ID on the outbound leg if misconfigured.
- Diversion (RFC 5806, historic) and History-Info (RFC 7044) record that a call was retargeted and why.
- A 302 from an endpoint is visible to the previous SIP hop. A PBX that hairpins may never send 302 toward the SIP trunk.
- Loops: A forwards to B, B forwards to A. Hop counts and diversion counters exist because this happens.
Sequential vs simultaneous
| Pattern | What rings | Vs a hunt group |
|---|---|---|
| Single forward-to | One number (desk, cell, or another DID) | Forwarding is one target. A hunt group is a list of members with a hunt algorithm. |
| Sequential (find-me) | Desk, then cell, then voicemail, each with its own timeout | Same idea as sequential hunt, usually owned by one user rather than a team. |
| Simultaneous (ring all) | Desk and cell ring at once. First answer wins. Other legs are cancelled. | Same idea as a simultaneous ring group limited to that user's devices. |
Simultaneous ring to a mobile uses an outbound originated call. The mobile's caller ID may show the original caller (if the platform passes ANI) or the company number. Passing the original caller is better for callback. It can also trip the mobile carrier's STIR/SHAKEN policy if the platform is not allowed to assert that identity.
Cell vs desk
- Desk / app
- Usually on-net: a SIP registration or WebRTC session. No second PSTN hop. E911 follows the MLTS / registered location for that device.
- Cell (PSTN forward)
- A second call onto the mobile network. Audio quality now depends on two legs instead of one. The mobile user may see a different caller ID. 911 from the cell is the mobile's 911, not the office MLTS path.
- Do not chain blindly
- Desk CFNR to cell CFNR to another desk creates long ring times and missed queue SLAs. Prefer one forward policy per user, then overflow to voicemail or a queue.
When forwarding is the wrong tool
- Team overflow belongs in a hunt group or queue, not a pile of personal CFU rules.
- After-hours should be a schedule (CFU to voicemail or an answering service), not hope that CFNR timers add up.
- An IVR "press 1" that then CFU-forwards to a personal cell hides the business number from the called party unless caller ID is preserved.
Recording, identity, and 911
If the forwarded-to party is outside the original recording domain, recording and consent may not follow the call. TCPA still applies to outbound sales legs. Emergency calling: a forwarded office call that the user answers on a cell is not an E911 call. 911 must be dialed from the device that needs dispatch.