There is a device called a Viking Rad-Amp that might be what you are looking for. It requires 2 phone lines at home, land lines or VOIP lines. They are connected by to the Rad-Amp by standard phone plugs. A call from your cell phone to line 1 is answered by the device and then connected to line 2 which then gives you your dial tone. I have one of these devices but have not used it recently so I am unclear about details. There is a pdf copy of the manual available on the Internet. You would be interested in CALL EXTENDER mode. As mentioned above, a Linksys SPA-3102 may also be able to do what you wish.