Android M 6.0.1.
Zoiper always fails to re-register after a network connection was dropped.
I switched off Wifi (to simulate leaving the house) and back on after about 5 min.
Zoiper is never able to re-establish a connection to the server, until I run the app manually.
Sipdroid for example, re-connects within a few seconds, after the network connection is available again.
I definitely prefer Zoiper over Sipdroid, but with this behaviour, Zoiper is just unusable for me.