Possible to change SIP message via proxy?

I have been doing a bit of research on this and think it's possible, but just wanted some confirmation from those in the know :-)

My requirement is to change the XML string in a SIP message body which is coming from a PBX server (closed source) and going to a particular type of handset. Problem is that the handset doesn't like the format of the XML, so I need to change it between the PBX and the phone.

I started out looking at changing things at the network layer, but quickly came to the conclusion that this would be extremely difficult (and my C/C++ is not that up to date), so I then turned my attention to SIP proxies, of which there are several really good open source ones.

But, bef开发者_开发知识库ore I head down this track, which may benefit others if possible, I wanted to ask, is this type of SIP message body manipulation even possible?

Thanks in advance for any responses :-)

From a standards perspective, proxies are allowed to modify the messages they pass between user agents. What you will need to do next, is find one that is programmable and allows you to modify the messages in a way that you see fit.

One option is a Back-to-Back User Agent, or B2BUA. The PBX calls your B2BUA which alters the message body appropriately before calling your handset (and in the reverse direction, of course).

(This might not be so trivial if you're using S/MIME to encrypt message bodies!)