How would one mod a scanner to receive 800-900Mhz frequencies to listen to cell/cordless phone calls?

some you can, go to a mod site, or buy your scanner from Canada.

If you're in the US things have changed dramatically since cell phone freqs were allowed to be inputed to a scanner.

Today we have digital systems very very very few analog radio systems.

These new digital systems are trunked. Just to name a commonly used system (IDEN).

What im trying to say you're not going to get anything.

Best bet you do is make images.

98 PERCENT of the US's phone system are untrackable due to being digitaly trunked. 2% are analog. IMO its worthless. Talk to someone who actually monitored cells when they first came out and if they try to now they only get 1. either NOTHING 2. or very few conversations.

EDIT: You can still listen to 900mhz cordless. Some Vtech.

Quite true. Very few phones are analog. I had an old bag phone that you could use as a reciever, and though you could get some analog conversations, the majority were digital. And for cordless phones, you won't have much there either: most are in the microwave spectrum...