In my days of work, I ran the hardware teams designing TETRA hand portables. I actually knew quite little about TETRA as most was implemented in software. What I knew I have forgotten!
These days, you can get freely available software that can decode modes like P25, DMR and TETRA.
TETRA was very popular with security systems as it was hard to intercept. Not any longer.
Southgate News has a piece about the closure of the Airwave system, used by police and ambulance services with TETRA radios in the UK. It is reported that this will stay operational until at least 2024 because of delays to the system due to replace it.
When I worked for Sepura (I retired in 2008) the UK police TETRA radios were an important part of our business. I am not sure what system they plan to use going forwards. I have completely lost touch in the last 5 years.
See http://www.southgatearc.org/news/2020/october/airwave-closure.htm#.X314akjYq00
