When I try to use Zoiper on my own WiFi network, it cannot connect to network. I even added it as an exemption to my VPN application (DNS66).
A year ago in beta it was definitely able to connect to my SIP provider (with the very same VPN program), but since around last November things broke and I cannot even connect. The desktop version works.
Some debug logs which might be interesting for you:
DEBUG | 20191007-071758.859 | RESIP:TRANSPORT | 520238380272 | InternalTransport.cxx:121 | Creating fd=79 V6/UDP
DEBUG | 20191007-071758.860 | RESIP:APP | 520238380272 | route_helper.cpp:283 | connect error: 101
INFO | 20191007-071758.860 | RESIP:TRANSPORT | 520238380272 | Transport.cxx:200 | Some other error (101): Network is unreachable
DEBUG | 20191007-071758.860 | RESIP | 520238380272 | BaseException.cxx:21 | BaseException at /home/autobuilder/build-jniwrapper/jniwrapper/dep/library/WRAPPER/source/route_helper.cpp:286 Can't find source address (UDP/connect)
INFO | 20190929-192331.304 | RESIP:DNS | 519628125424 | AresDns.cxx:533 | DNS initialization: found 3 name servers:
ERR | 20190929-192352.585 | RESIP:DNS | 519628125424 | DnsStub.cxx:609 | Connection error Timeout while contacting DNS servers for sip.stuntcalls.com
DEBUG | 20190929-192352.585 | RESIP:DNS | 519628125424 | DnsStub.cxx:49 | Host(A) Result: sip.stuntcalls.com lookup