You'd need to get the secret info on the frequency hopping scheme as well as the shared encryption key. I've heard of units being ripped out of police cars for this reason.
I meant dialup modem tapping. 2600 or phrack had something about feds being able to wire into telephone line and record/live decode modem connection. You could easily write something like this in GNU Radio for <=2400 links (or use minimodem), but anything higher transmits full duplex on same frequencies - both modems mutually jam each other.