A method of roaming between a mobile network and a wireless network can include receiving, over the wireless network, an invite from a user agent disposed within a mobile communications device. The mobile communications device can be engaged in a call over a voice channel in the mobile network. The method also can include selecting one of a plurality of gateways having an affinity with the user agent, wherein each gateway is configured as an interface between at least one mobile network and the wireless network. The call can be handed off from the mobile network so that at least a portion of the call is conducted over the wireless network using the selected gateway.