CTI attended transfer to external number misses isTrunk SIP header, enabling SRTP and causing 488 from provider

[Stell0]

When performing an attended (consultative) transfer to an external number via CTI (Phone Island / /astproxy/atxfer) generates an outbound INVITE from the proxy with SRTP/encryption because the isTrunk SIP header is not present. The provider rejects the call with 488 Not acceptable here.
The same transfer executed from a physical phone works, and the INVITE includes isTrunk: 1.

Steps to reproduce

1. Configure an external trunk that requires SRTP encryption disabled (isTrunk: 1)
2. Place a call involving an internal user/extension.
3. From Phone Island UI, start an attended/consultative transfer to an external number (CTI/server-side transfer via /astproxy/atxfer).
4. Observe the outbound INVITE sent by the proxy does not include the isTrunk SIP header and carries encrypted media (SRTP/crypto).
5. Provider replies with 488 Not acceptable here and the transfer fails.
6. Repeat the same scenario using a physical phone to transfer to the same external number.
7. Observe the INVITE includes isTrunk: 1 and the transfer succeeds.

Expected behavior

CTI transfers to external/trunk destinations should follow the same outbound/trunk call path as phone-originated transfers, ensuring isTrunk: 1 is present (or equivalent behavior), and the provider accepts the call.

Actual behavior

CTI attended transfer to an external number does not set isTrunk, resulting in encrypted media being offered and the provider rejecting the INVITE with 488 Not acceptable here.

See also

• https://mattermost.nethesis.it/nethesis/pl/ius66emkyfbt9fndao1t9ejoba
• https://helpdesk.nethesis.it/a/tickets/202231
• Nethvoice Proxy: Compatibility issue with initial SDP Offer using SRTP from NethVoice Proxy #7524
• NethVoice Proxy: issue on re-INVITE when INVITE with RTP cames from a device that doesn't support SRTP #7546