When it happens
- Require: 100rel, timer or replaces sent to a device that does not support it.
What to check
- Send the option in Supported instead of Require, or disable it (for example PRACK/100rel) on the trunk.
What the response looks like
SIP/2.0 420 Bad Extension
Via: SIP/2.0/UDP 203.0.113.10:5060;branch=z9hG4bK776asdhds
From: <sip:101@pbx.example>;tag=1928301774
To: <sip:+15551234567@pbx.example>;tag=a6c85cf
Call-ID: a84b4c76e66710@203.0.113.10
CSeq: 2 INVITE
Reason: Q.850;cause=127;text="Interworking, unspecified"
Content-Length: 0