I have used Zoiper Pro with push service (Huawei P9 EVA-L19, Android 6.0 / EMUI 4.1.1). Every incoming call was interrupted after exactly 32 seconds (confirmed by my billing of my VoIP-operator). Additionally: call duration of incoming calls in call log was every time 0 sec.
On the same circumstances Zoiper Free was working fine and stable (the same VoIP-account, the same Zoiper-settings, the same mobile network).
Now is the one month subscription over, the push service is no more active and... Zoiper Pro is working fine - no more interrupted calls and the call duration in the call log is always correct.