Custom SIP invites

I was wondering how to build custom SIP headers. I see the ‘sipAddHeader’ option available but it does not provide the level of customization I need. I would like to manipulate the INVITE message being sent. It will be used for testing and can be a static parameter. For example, I might want to try to add cic=0288 in the sip uri. Or add tgrp=123 the TO field. Somewhere in the asterisk code its builds the INVITE. If I knew where the file was, I should be able to modify there. Any guidance to point me in the right direction would be helpful. Thank you very much,

There aren’t any user handles for manipulating the SIP messaging to that level, so yes, you’ll have to dive into the code. Asterisk’s SIP channel driver is maintained in the: