Handle multiple proxy authentications
When sending an INVITE
we might need to authenticate not only to our own SIP server, but also to additional servers/proxies (see https://tools.ietf.org/html/rfc3665#section-3.3).
telepathy-rakia f.e. has EXTRA_AUTH_USER
and EXTRA_AUTH_PASSWORD
properties: https://github.com/TelepathyIM/telepathy-rakia/blob/master/src/sip-connection.c#L86
Edit: I think this is rare enough to not give it a huge priority right now