SIP Call Transfer Between FreePBX 17 and Microsoft Teams via Audiocodes SBC — A Quick Deep Dive
In a recent lab validation, I tested a full SIP call flow involving FreePBX, Microsoft Teams Direct Routing, and an Audiocodes SBC operating in B2BUA mode, with a Twilio SIP trunk as the PSTN breakout. The goal was to validate a seamless call transfer initiated from Teams back to FreePBX, and ensure proper SIP signaling, media continuity, and number normalization. Architecture:
Call Scenario:
A user on FreePBX extension 1002 initiates an outbound call to Microsoft Teams, routed via Direct Routing and Twilio.
The call reaches Teams, and the user performs a SIP REFER transfer to another FreePBX extension (1001).
The Audiocodes SBC handles all SIP signaling and media anchoring between platforms.
An outbound manipulation rule rewrites the SIP To: header from internal extension 9401 to a valid PSTN number, allowing the call to exit via the Twilio trunk.
Infrastructure Roles:
Call Flow Summary:
INVITE from FreePBX (1002) to Teams via SBC
(taken from Asterisk)
Teams initiates REFER to transfer the call to FreePBX (1001)
SBC relays REFER → 202 Accepted → NOTIFY → 200 OK
BYE and ACK exchanged cleanly
Outbound manipulation rewrites To: 9401 → +33XXXXXXXXX First Invite to SBC :
Second Invite sniffed out of the SBC :
all exits via Twilio SIP trunk
SIP Behavior Observed:
REFER fully supported by Teams and SBC
NOTIFY messages correctly acknowledged
No SDP or codec negotiation issues
RTP remained anchored and stable
SIP headers normalized post-routing
Outbound manipulation applied after IP-to-IP routing, ensuring proper PSTN formatting
Audiocodes SBC Highlights :
Proxy Set configured for FreePBX with keep-alive
IP Group validated with REFER support
Routing Rules defined for both directions
Outbound Manipulation Set applied to rewrite internal extensions to PSTN format
SIP Interface and Media Realm tuned for FreePBX compatibility
Conclusion:
This test confirms that a properly configured Audiocodes SBC can handle cross-platform SIP transfers, number normalization, and media continuity between FreePBX and Microsoft Teams. The REFER flow was clean, the manipulation logic worked as expected, and the call exited via Twilio without issue.
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