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.
Excluded from the investigation but still kept and also recorded in the first place because of dragnet surveillance. If I'm not comfortable with them seeing anything because I'm not guilty of anything what do I do then?
Which is a strange thing to do with an innocent person's data but that is what's reported.
A few years back people considered "H.R.4681 - Intelligence Authorization Act" to be a victory in terms of limiting how long government can keep information of U.S. citizens "not wittingly"[1] captured by intelligence agencies in their hoovering activities. I thought the coincidence of the specific provisions in that law with the drive towards making sure all communications on the web is encrypted was "interesting".[2]
[1]: https://fas.org/blogs/secrecy/2014/01/clapper-ssci/
[2]: https://www.nu42.com/2014/12/https-everywhere-and-hr4681.htm...
Rough translation of parts of the header of https://www.vrt.be/vrtnws/nl/2021/03/11/oproep-sky/ :
I don't know how to translate [veiliggesteld] as it is pretty vague. The word might be approximated as 'made safe' or 'put in a secure location', but it surely does not mean destroyed.As expected, the whole thing raises some eyebrows in privacy circles.