Hello,
We are working on implementing the iOS CallKit, which should solve the issue with the ringtone. The strange thing is that this issue is actually very intermittent, but with the availability of the the CallKit the behaviour of the incoming calls should be almost the same as the regular GSM ones.
Currently we cannot provide ETA on when the CallKit will be available, but we plan the implement it in the next Zoiper Premium release and probably the next Free release or the one after that. Once available it will be listed in the iTunes AppStore changelog for the version.
In addition, please also note that if you use one account on multiple devices the server should handle that and must send the proper package to the devices. Other than that Zoiper is using the native notification service of iOS, because of that you are receiving a notification for all events send from the system to Zoiper.