Keep in mind the the use of one account on multiple devices must be support by the server as well - more details on that matter cane be received from your VoIP provider.
For the calls receiving, first make sure you receive them when the app is opened, if not try to alter the STUN settings in the Network settings section from the account configuration. In case the calls are not received in background, the best way for receiving incoming calls while Zoiper is in background is to use TCP or TLS instead of UDP. This could be set from the Network settings menu in the account configuration. Please also be advised that TCP and TLS must be supported by your VoIP provider as well, otherwise your account will not register.
If for some reason, you are not able to use anything else than UDP, you may enable both options in the Incoming calls section from Zoiper's Settings menu.
In addition, please also check whether all notifications are enabled for Zoiper in the iOS Settings menu -> Notifications.