Do you tell me this because of theoretically reasons or do you have real problems with that?
I have less problems in reusing existing connections - the reason is NAT! Using the same connection from registration for outgoing calls works without any problems with providers supporting TCP. Incoming calls from these providers are also signaled via this open connection. So the way PhonerLite works is not a proprietary one - there are a lot of implementations using this connection reuse.
So if you have a real problem with that - just tell me!