Leech - Ultimate Knowledge Group

Old but good plans from gbppr. Frequencies may need to be adjusted to block today's phones. Will work against other devices as well. Build at your own risk!

http://gbppr.net/mil/celljam/index.html

Good link madmax, this is something can be applied to jam several technologies if the frequency is varied, such as GPS, drones, WiFi, etc

This is an oldie but goodie - can be modified to jam today's phone frequencies. May need more powerful amps though:

https://www.ladyada.net/make/wavebubble/