Are you using a VPN ?
What version of android are you using and what model of phone ?
Does it eventually re-register within the next 10 minutes ?
We have investigated similar reports in the past:
The network is probably not completely up when we try to connect after receiving the network change, the registration fails.
This happens for example if you use a vpn and zoiper is informed about the network change, but the VPN is not up yet and once the vpn is connected, there is no new "network change" event.