Proxy across transport in SIP

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First reported on Jul 2, 2012 by Debasish Chandra <[email protected]> 
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However I noticed that sipd, only allowing call between phone register 
webscoket or between phones which are not using websocket. It doesn't 
interconnect between normal phone register using UDP with phone using 
websocket. Even though its getting proper port and IP, but using the same media 
as originating phone. For example if any ph connected with websocket, and when 
try to call another phone registered using UDP, SIPD trying to connect with 
that phone using websocket only. In case if it is originated from a phone using 
UDP transport, it sending Invite over UDP only, not on websocket.

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The way sipd.py (and sipapi.py) evolved it was not trivial to change the 
transport on proxy. Need to re-think the design of the voip.py's Proxy class so 
that it allows multiple transports.

Original issue reported on code.google.com by [email protected] on 9 Jul 2012 at 8:50