As far as I understand, they were logging the encrypted messages for a long time, but only had live access for a few weeks. My guess is they used this to intercept a large shipment (17+ ton of cocaine captured), and watch the communications after that to rapidly identify the players involved.

Sky phones were set up with dedicated sims, everything else but their app blocked. There's no telephone number, email adress or other id to identify users. The app also featured self-deleting messages and post-seizure remote wiping. Besides the phone, they required an expensive subscription.

If these phones keep showing up on suspects, only available via no questions asked dealers, and you can't get anything from the phones themselves, it's easier to get a court order to go after the service itself. If you can then log all encrypted messages on or near the servers, and see they're mostly going to Belgium/Netherlands, then you can try to read all messages. Either via decryption, or compromising update servers, or ...

Because there's no way to identify users besides reading the messages, they asked legitimate users to come forward so their data can be excluded from the investigation.