Everyone is familiar with the likes of WhatsApp and Telegram for private, encrypted chat services, but have you ever heard of EncroChat? Me neither, but then I'm not a criminal. Turns out that this service was created exclusively to let criminals talk to each other on specially modified Android phones using encrypted chat. Of course, the prices for this service were in the thousands. The identity of the founders of this service have proved elusive and thankfully it only lasted for 4 years before the police busted it, so it's now defunct. What all those nasty criminals are using in its place I don't know, but I can tell you that it was far more secure than WhatsApp.

Incredibly, the website is still active, but of course, not for its original purpose. Clicking the link there takes you to a very detailed page about the service and how it was busted. Note that there's no preview for the site, so you just see the URL.
en.wikipedia.org

EncroChat was a Europe-based communications network and service provider that offered modified smartphones allowing encrypted communication among subscribers. It was used primarily by organized crime members to plan criminal activities. Police infiltrated the network between at least March and June 2020 during a Europe-wide investigation. An unidentified source associated with EncroChat announced on the night of 12–13 June 2020 that the company would cease operations because of the police operation.[1][2]
The service had around 60,000 subscribers at the time of its closure.[3][4] In the UK the National Crime Agency led an operation resulting in over 2,600 arrests and 1,384 criminal charges.
Incredibly, the website is still active, but of course, not for its original purpose. Clicking the link there takes you to a very detailed page about the service and how it was busted. Note that there's no preview for the site, so you just see the URL.