Same issue here, but Android 6, on an LG G3.
Using the pre-configured SIPGate install, registers fine, and I can make calls to the SIPGate voicemail service and their test numbers.
Sipgate's web site also successfully lists the phone as registered with the correct port number.
However even when running on wifi at home (so I know it's not the mobile network blocking SIP packets) I'm not receiving any calls on the Android phone, when my other PCs on the same network do receive calls.