Given that tetra is a cellular based architecure then even if you could crack it, you are unlikely to get everything unless all parties on the comms your listening to happen to be in the same cell
Geographically it doesn't matter as the network is national, we use them on occasion and have people down in Portsmouth talking to us while we play on the Plain 
It would seem the best way is to clone one, so you'll have to drug one when they come in Di then nick his radio and clone it...... or just seduce the bugger

Your missing the point....as its a cellular based setup, unlike the wan type radio that was used, you can only get transmissions to and from those that are in the same cell as you if you are trying to listen in.
The cells are considerably smaller than the old radio setup and hence much less traffic in each.
tetra supports both multicast and point to point wih most calls being made with the latter (this is the key, its closer to a phone type setup than a broadcast one)