Hi, i have a similar problem with Android 5.1.1.
Here its the respons i have from the developers by mail:
"The reported issue could be related to a failure in the signaling caused by NAT issue. Try to alter STUN and rport settings in Zoiper and make sure nothing is blocking requests/responses."
This fix the problem for me:
Go to settings / acounts / choose your acount / Network settings / and there check both RPORT options they are on the bottom of the setting list.
I hope this fix your problem to.