If you do not recognize the term cellphone, s/cellphone/mobile/.
Many people are reluctant to relinquish their landlines, and in some cases, for good cause. Here are the (reasonable) reasons i know of:
1) landlines can sometimes work in power outages when cells won't
2) the reception is better,
especially when the user has wireless deadspots in their home and likes to walk and talk
3) Landline headsets are more comfortable than cellphones*
3b) *bluetooth headsets make people look like tools.
4) there are usually phones in most major rooms in a house, but the user would have to carry a cell around with them if it were their only phone -- or go running for it -- or not hear it if it is far away.
This product helps to solve part of #2, and all of 3 and 4.
It includes a base which has bluetooth, and plugs into the existing phone wiring of the user's home, as well as a standard mains outlet. The unit should be positioned in the house at a point of maximum cellular reception quality.
The user sets the cellphone atop the device, which pairs with the phone's bluetooth. There is also a jack coming out of the unit which plugs into the phone to keep it charging. The user may forgo this charging and increase distance between the unit and cellphone, but since bluetooth drains batteries at a faster rate, it is recommended the phone be plugged in somewhere within acceptable bluetooth range.
The unit powers the home phone circuit, keeps it open, and emulates a dial tone (just so grandma won't be confused).
Did I mention that It has a DTMF decoder as well?
The user may now pick up any phone in the home (cordless phone for extreme irony), then dial a number, which will be interpreted by the DTMF decoder in the device, and sent to the cellular phone to make the call using the cell phone's connection.
Similarly, when a call comes into the cellphone, the unit will be notified by bluetooth, and make all the phones in the house ring. Picking up any of the phones will cause the call to be answered and the conversation will take place over the landline.
Speed dialing from the cellphone's memory will be supported, and the interface superimposed on a regular telephone keypad in one of several user-selectable ways. i.e. if the user wants, picking up a landline handset and pressing #1 will dial voicemail, #2 will dial the local shopkeep, and #3 will dial your psychiatrist, as the user clearly spends too much money on gadgets, and is obviously compensating for a lack of happiness in life.
The unit will also transmit any voice before dialing, so if the phone supports it, dial-by-voice will work properly.
The deluxe version will pass caller ID data over the house phone lines so smart(landline)phones with displays will still be as useful for avoiding your in-laws.
The manual will mention that if the user has a home-answering-machine, it should be disconnected, lest the messages wind up in the wrong place at the wrong time.
I would be surprised if this didn't already exist, but I am at a loss to find it being sold or described.