If what you wanted to do was get a PC to talk to a remote phone socket what you would need is something like a DECT Modem.
However I'd not rush out an buy one as I do not believe it is the solution for a Sky to socket link as the sky box has a built in modem, so what it needs to see is a line and a dial tone, not a modem serial port.
There may be some sort of DECT line extender you could get.
This might shed some light on why they are so keen (after the required 12 months) to keep you connected to a phone line:
http://www.e-repair.co.uk/story146.htm
And this one, where having 2 boxes is mentioned as being a special case needing a connection all the time:
http://forum.digitalspy.co.uk/board/...d.php?t=192359
Looking at the two above links it seems that if you plug each box it into the phone line once in a while Sky will stop hassling you as they can download the data they want.
The phone home function is one of the reasons I have not bought Sky.