Hi Rick,
(Nils: Thanks for your helpful reply.)
unfortunately the usage of 305 is totally under specified. And I haven't it being used in the field ever. Anybody else maybe?
There are good reasons for using it though. For me, these are:
I want to pass off any POTS calls to upstream SIP providers without having to worry about RTP proxies (setting them up, or wondering if they would be needed at all). If traffic goes through an outside-NAT proxy, the sip2pots provider would probably decide no RTP proxy is needed.
I want to avoid being responsible for the traffic that I pass on. This means that I can be a proxy to unregistered users, if they like. It makes it very easy for them to try out a service if they only have to set the "proxy" field in their phone.
I understand that all this reasoning won't help to improve the RFC, as this is not the task of the people on this list. I just wanted to clear up why this is useful (to me at least).
sure, I totally agree that there are good use-cases for this response. But the problem seems to me that you will have to teach every SIP element in your setup how to treat this response correctly (like you expearienced already with your Grandstream phone). Good luck with that. And I would be really interested if you make progress on that front. BTW there were some discussion about this response on the IETF mailing list several years ago. But they somehow died.
Regards Nils Ohlmeier VoIP Freelancer