There are two ways to await for incoming SIP calls:
1) use Google Cloud Messaging (GCM) or APNS (for iPhone) for wakeup and then establish SIP connection to intermediate server. Obviously this requires intermediate SIP proxy server to know your SIP credentials.
2) establish usual TCP/IP SIP connection and have the phone to awake each time a keep-alive packet arrives, thus draining battery. Also need to bear with reconnections, especially as the phone travels between networks or its IP gets changed.
What method does ZoiPer use?