What kind of handsets (even better if you know the models) were you using and were you using any headphones/microphones?
With headphones/microphones the plugs and sockets might look alike and appear to fit but may not be compatible, and also some may only work when pressed in hard (in which case the plastic on the plug can be shaved slightly)
There are multiple analogue and digital PMR frequency’s, with 16 original analogues, 8 then 16 digitals
and then a newer set of 16 to 32 digitals
If you mix and match the different versions then it should not work as the actual frequencies differ
There is also scrambling, but if enabled you should not hear anything if you cannot unscramble
And with the very old cheap versions of digital PMR - a cheap one would only have 8 channels and a fully featured one would have the 38 sub frequencies within the channels.
E.g. channel 3.00 would be different to 3.01
But the cheapest ones would transmit on 3.00 but be able to hear every sub frequency from 3.01 to 3.38
Someone with a handset that supports sub frequencies could only hear true 3.00 and would ignore 3.01 to 3.38.
I used to like to hang onto these types when running games to monitor multiple sub channels
https://kenwoodcommunications.co.uk/files/file/comms/uk/pmr446/PMR446-White-Paper-V6_18AUG2016_JT_KB.pdf
