Page group with Softphone?

I am attempting to use a Softphone (written by our company using an SDK), and I setup the extension to be part of the paging group. When I dial 8000, the phone that dial gets a tone that is ready, but the softphone never seems to get an INVITE. I went through the autoanswer dial plan and it seems that the user-agent has to be one of the pre-defined types for it to be sent the autoanswer headers. Is there a way to add additional user-agents rather than setting the user-agent in the softphone. Also, will this affect the call to the extension? If not, any suggestions on why the softphone extension is not being called?