Call transfers weren’t forwarding calls to extensions, only to the main phone. It’s working now. Can someone tell me what changed to make it work? We had lines connected to Zadarma that worked, and lines with other companies’ free-PBX systems that didn’t work. Now they all work, and the free-PBX systems that weren’t working before are now working. Another thing is, could someone please explain why they work with Zadarma but not with others?
Based on the available sources, Retell AI can only transfer calls to full PSTN numbers or SIP entry points, not directly to internal PBX extensions(1). The system transfers calls using either cold transfer (direct transfer without interaction) or warm transfer (AI briefs the next agent before transferring)(2)(3).
The key limitation is that Retell cannot transfer directly to internal extensions(1). The recommended approach is to transfer the call back to the PBX via a dedicated DID or SIP trunk, then let the PBX route internally using IVR logic, language flags, or call context(1).
For your situation where transfers now work with free-PBX systems that previously didn’t, the likely resolution involved one of these changes:
PBX Configuration: Your PBX may now expose routing numbers (DIDs or equivalent) that map to specific extensions or queues(1). When the AI hands off, the call is sent back to one of those numbers and the PBX handles the internal routing(1).
SIP Header Support: If your PBX supports SIP headers, you can pass intent or language metadata and route dynamically(1). Retell supports adding custom SIP headers for outbound calls, which are forwarded to your SIP provider on the SIP INVITE and can be used for custom routing, tagging, or metadata(2).
Regarding why Zadarma worked while others didn’t: different telephony providers have varying support for SIP trunking features(4). Some providers may have concurrency limitations that prevent transfers (which require 2 concurrent calls)(5). The provider must support the specific SIP features needed for call transfers to function properly(4).
For cold transfers specifically, Retell uses SIP REFER, and caller ID override support varies by provider(2). Retell Twilio numbers support showing the user’s number on both warm and cold transfer, while Retell Telnyx numbers only support this when using SIP REFER via cold transfer(2).