Please switch to TCP instead of UDP if you can. It will use a lot less battery and will be much more reliable.
The running in background with UDP is not officially supported by Apple and is a bit of a dirty hack for servers without UDP support.
When using TCP, Zoiper will hibernate and iOS will take over the TCP connection, will reply to TCP keep alives and will not wake up the application unless a SIP message is sent to zoiper.
Each 10 minutes Zoiper will wake up, update the registration to the server and will go back to sleep.
Additional benefit when using TCP:
If the application crashes or gets killed by iOS or the phone is rebooted, Zoiper will automatically restart when using TCP.
If the Application is woken up too many times because of incoming SIP packets, iOS might decide to kill the application to preserve battery. To avoid this, please avoid using SIP keep alive packets (qualify on asterisk) or at least dont send them too often.
If you cannot use TCP, for now it is not possible to "hide" the app completely while in background, but it is technically possible and i will discuss it with the development team to add this for a future release.