A system and method implemented at a communication device for providing access to telephony services over one or more of a number of communication networks, including cellular networks and IP-based fixed and wireless networks. A mobile device may be configurable for voice communication over both a cellular network and a LAN (e.g. a wireless LAN), and may be adapted to place and receive calls via a PBX over one of the cellular network and the LAN. When it is determined that the mobile device is configured for voice communication using one or both of the cellular network connection and the LAN connection, the mobile device may automatically adapt its user interface to reflect the availability of voice communication only over those connections that are available.