Hello. I am George. I have read your post and really attracts to me.
Your requirement is to make softphone just like X-Lite. I worked for the private VoIP operation company so that backend system such as 3A with Radius and FreeSwitch and front end system just like X-Lite, PBXManager really no problem.
But this is not small project so that if you can pay me 3000$ for this project I will try.
I am full of confidence of your project so that all we have to do is to discuss with the budget.
If you are interested in, please message me.
Regards.