Which version of Zoiper for Android you are using ?
It's ZoiperBeta 2.4.3
Can you install the latest Zoiper Beta and check if the same behaviour would persist ?
What is the phone brand and model ?
I installed the latest one, 2.5.30 and it's reproducible with that one, too. It's a Sony F5121 (Xperia X). The "Phone" application has the version number of 3.00.00. Android: 8.0.0.