That’s one case. It doesn’t apply to all criminal organizations. The cartels in Mexico are sophisticated enough to build their own cell networks [1] to evade wiretapping. Why couldn’t they also recruit engineers to build their own crypto and secure protocols?
that's not a cellphone network that's a VHF/UHF radio repeater network. basically same idea as ham radio hilltop repeater stuff in the US, but built for private purposes, and using COTS radios capable of basic encryption.
there are also cheap vhf/uhf handheld radios from china which integrate a basic 2G GSM radio, SIM card slot, and can talk to common euro/asian 3GPP radio bands.
[1] https://www.npr.org/2011/12/09/143442365/mexico-busts-drug-c...