The Cymbus app uses the SIP protocol for all its telephony functions. When you configure a SIP account in Cymbus, it uses that information to register with your VOIP server, or PBX. When someone attempts to call you, the call goes to the PBX first, and, if Cymbus is registered, the PBX routes the call to the softphone and which will ring and alert you to the call.
This can present a problem for mobile softphone clients. Modern mobile operating systems are generally very aggressive in terms of resource management. When an app is sent to the background or the phone is idle, apps like Cymbus are likely to have their network connections closed, which means the PBX will not be able to reach the app when a call is received.
This is where push comes in. When push notifications are enabled and Cymbus is sent to the background, it unregisters your SIP account from the PBX and sends a signal to one of our push servers to register the account on the app's behalf.
With the push server registered, the PBX can now route incoming calls there instead, and the push server is able to remotely wake the app via the Apple or Google Push networks.
The client (softphone app) can then be opened to receive the call.