Hello,
Please open your Zoiper on foreground and use the side buttons of the device to increase the volume.
Most probably the issue you experience is related to the use of UDP transport type, since it is not reliable on iOS.
You would be able to change the transport to be over TCP or TLS, but your VoIP provider have to support it, otherwise the account will not register. As for changing the transport, you may do that from the "Network settings" in the account configuration.