No... GSM is unsecure!!! Stop thinking old = simple therefore secure. It's not.
A smartphone with a VPN and secure messaging app / voip will always beat anything else. Then if you mix in IMEI changing etc it will be even better. Believe it or not, the phone is NOT the biggest issue. The telecom part is. Everything else you can just kind of shop around and get the best.
There are 100% secure communications... it's called encryption.. the problem is not in the communications but the technology of how you connect to the internet, there's a lot of ways the authorities can track your regular telecommunications. IMEI SIN etc all are tied to some kind of database listing SOME info. Even if you get a
prepaid card that will narrow you down to some country or region or even city. And if you're using that same device well now its connecting to the nearest cell tower and they have a pretty accurate location of where you are. Point is, you should make it so that it's not possible for them to even get to the point where they can get enough info to start tracking you.
Btw, with all of your tails setups etc, it may be surprising how they actually track you. For example, your style of writing, your interests that you may reveal here etc. Its all quite the things you don't pay attention to. For example, I see you have a habit of CAPITALIZING your words to emphasize them. Or how you use the word "Comrade". This would be something that one would use to narrow down "suspects". The FBI caught a darkweb forum admin in this way. They found him because he used the word "hiya". He forgot to lock his laptop for a minute and the FBI had all their hands on it.