Since iOS 11+, Apple no-longer allows apps to work on background, thus when
you put Zoiper on background the operating system is putting it to sleep. We had a workaround that used to keep Zoiper working on background, however
since the the iOS 11 updates, it is no-longer reliable.
The only workaround would be to enable the PUSH notifications. You can find the option under Zoiper -> Settings -> Incoming calls.
However, if the timeout of the server is too low the PUSH may not work reliably.
For reliable PUSH notifications we suggest at least 10 minutes of registration expiry time, or if possible the server to allow up to 1 month.
You can also find more details regarding the Zoiper PUSH service, here: