I don’t know whether you want to upgrade your fixed phone to act like a wireless device or not. But if you want to do that, edevice offers you an analog-to-wireless converter device, WireX.
The WireX allows your buit-in analog modems fixed phone to connect to your cellular packet data in easy way. You just need to unplug the phone jack from the wall socket and plug into the WireX traditional dial-up PSTN network Emulation Input socket without require any change to the host.
Dressed in compact size: 120(W)x103(D)x36(H), WireX features RJ11 POTS/FXS interface, an internal antenna, a SIM card holder, an internal Li-Ion battery, and LEDs indicating GSM signal strength. Thanks to wall mounting casing, it enables you to place it in your wall perfectly. [Dandy Gadget Source]