The present invention is directed to systems for and methods of using dual mode handsets or softphone client for voice, sms, and data services. In one embodiment of the present invention, a mobile handset uses a SIP User Agent to register on a visiting network. The mobile handset generates SIP REGISTER messages. The SIP REGISTER messages are translated into corresponding MAP registration (or RADIUS message) and authentication commands, allowing system to contact the HPLMN HLR (or home AAA) associated with the mobile device to authenticate the mobile device and register it on a VLR of a visiting network. MAP responses (or RADIUS response) are translated to corresponding SIP commands that are forwarded to the mobile device, thereby completing the connection set up.