When it happens
- Every INVITE without Proxy-Authorization gets a 407 first.
What to check
- If the INVITE with credentials gets 407 again, the username, password or realm is wrong.
What the response looks like
SIP/2.0 407 Proxy Authentication Required
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=21;text="Call rejected"
Content-Length: 0