Paging use the auto-answer feature on the phone so You need to check if your softphone support paging. And in the other hand you need to provide the sip debug to know whats happening at the moment of the call.
The retransmission issue is not about NAT is about the peer never ACKs some SIP Messages.
If the phone doesn’t respond at all, it is broken.
Most of these problems are NAT or firewall related, although I have come across soft phones that just ignore things they don’t like, rather than rejecting them.