Hi,
I recently got access to the possibility to use VOIP. I installed Telephone on my OSX. On my machine I also had installed X-Lite as VOIP client.
Using the Unicall workflow I selected dialing with Telephone. Nothing happened in Telephone, but X-Lite kicked in and dialed out.
So my conclusion is that X-Lite somehow took over the sip: scheme and was the default app.
There are some utilities to set the default handler for sip:, but I did not exactly found out to do this the easiest way.
Just wanted to let you know, perhaps this is by design. In any case you can probably change your docs saying that the SIP dialer will use the application that is set the default app on OSX.