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.

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