[notyourday]

> Properly installed (F-Droid) on off-the-shelf phones with fresh prepaid sims and OS updates disabled, it can be considered secure software against all but the most sophisticated adversaries.

IMEI will identify the phone.

Signal does not work well without GCM.

[yholio]

You can easily identify the phone of the target by physical surveillance coupled with Stingray type of cell traps. No need to bother with IMEI, plain cell number which you will then intercept with operator support.

So you will prove the target uses a phone, and that phone connects to Signal servers or some offshore VPN. That's not very useful.

The quest is to access communication contents, or map out a criminal network that can then then lead to identifying other physical locations of terminals and unknown members of the ring.