[SR-Users] Modern SIP Trunking, Route on To: or INVITE Headers?

[Daniel Tryba]

On Tue, May 21, 2019 at 03:57:02PM +0200, Benoit Panizzon wrote:
...
> So if the Voice Switch is sending back the Register Contact in the
> INVITE, the PBX cannot use this field to determine which extension to
> ring.
> 
> So it has to use the To: Header.
> 
> Well, not in a forwarding scenario, the To: header does not contain a
> valid extension, and the PBX will either invoke a catch-all or simple
> reply with 404.
> 
> So how to work around this?
> 
> Well, you cannot send the Registration Contact in your INVITE Request,
> but the INVITE has to contain the called extension behind the PBX. All
> PBX which I know can work this way, this is even mostly the default.
> 
> BUT, if you have an SBC in between, this is getting tricky, as the SBC
> usually identifies an dynamic endpoint by the user sent as register
> contact and expects the invite to contain that contact.
...

RFC3327 Path tries to solve this problem. If that "SBC" doesn't support
Path all you can do is send the Contact from the REGISTER and leave it
to the SBC to solve its own problems.