This wasn’t built to be a general purpose phone. It’s a single purpose tool, if it had full SIP support it would need more expensive hardware. When I worked on this I knew I wasn’t building some interoperable general tool.
I am being honest with you. For me the ‘hacker spirit’ means cracking things open and learning how they work. So I totally encourage others to do it.
I would be perfectly fine with taking "in theory yes but actually no" (which is basically what you are saying) as the answer instead of this roundabout way of phrasing it. I appreciate what you did, but this discussion is kind of unnecessary.