As a reference, I have a PAP2 connected with 2 different providers on line 1 and 2. I am able to call from either line to either line with no audio or disconnect issues. My Voipo PAP2T is connected to the same router, which leads me to believe the issues are not on my network.

I am also able to place and receive calls from my Grandstream IP phone, without issue. This phone is also connected to the same router as the 2 PAP2's.