A mobile communication device includes a wireless transceiver, one or more processors coupled to the wireless transceiver, memory, and a communications application stored in the memory. The communications application may be a VoIP telephony application which involves communication in accordance with a session initiation protocol (SIP). The one or more processors are operative to receive a public IP address assigned to the mobile device in a public network; cause a request for a virtual private network (VPN) connection with a private network to be communicated; refrain from registering with a SIP server for the communications using the public IP address; and if the VPN connection is established: receive a private IP address assigned to the mobile device for the VPN connection and cause a request for session registration using the private IP address to be communicated to a SIP server of the private network.