The user interface of the Zoiper app in iOS differs from the one on android.

If a connection with TLS enabled is not possible you can change the transport protocol to TCP or, preferable, UDP.

Your calls will then be unencrypted

Handling incoming calls on iOS can be challenging due to the operating system's strict optimization for battery efficiency, which suspends app activity when the app is not actively in use. To ensure that apps can receive incoming calls, iOS requires the use of Apple-certified push notification services to wake the app. In zoiper this service is a premium feature which can be booked for 0,99 € / months. A one week free trial can be used for the first chaos event. This feature has been tested in Zoiper Lite and works without Problems.