Android 2.3 (Gingerbread) and newer has a built in SIP/VoIP client. The widget has two buttons: The left one gives you a shortcut to the SIP settings i.e. to enable incoming calls. The second one allows you to select how calls should be routed primarily: you can select whether all calls should be made via SIP by default; only Internet calls via SIP (Airplane mode), or if you want to be asked before each call.