Just so no one gets confused, the whole cordless-phone/tv trick is for listening in on other cordless phones. This doesn't do anything for cell phones. You may be able to pick up an old analogue cell phone, but its doubtful. And like nine volt said, you don't have to use a tv. You could use a radio with your cordless. It also only picks up phones on or near your same frequency. You are not going to here a 2.4Ghz phone with that old 8Mhz phone thats stuffed in a box in the back of your garage.

Sniffing a cell phone is harder then this. Depening on the system, it may not be a lot harder, but still more work. I thought someone cracked GSM a while back though. Maybe it was one of the other digital encrypted phones systems. Oh well, just know that there is no such thing as secure phones, and you will be ok.