https://wiki.zerophone.org/index.php/Main_Page

[grarpamp at gmail.com (grarpamp)]

> https://wiki.zerophone.org/index.php/Main_Page

Open phones could also choose SDR instead of closed
source baseband. Form factor and cost there is higher
yet both can be made smaller by assembling a single
purpose SDR RF board.

There's also a lot of cheap SIM based alarm boards
out there to hack with, more or less like the SIM800L
serial uart modem scheme. Some regions cellular
networks may no longer support older than 3G 4G, etc.

https://duckduckgo.com/?q="sim800l";

https://img.filipeflop.com/files/download/Datasheet_SIM800L.pdf
http://www.sim.com/

http://openbts.org/hardware/

https://www.openairinterface.org/docs/workshop/3_OAI_Workshop_20170427/Session2_UE/Lin_Huan_-_UE_Security.pdf
https://www.openairinterface.org/

https://media.defcon.org/DEF%20CON%2024/DEF%20CON%2024%20presentations/DEFCON-24-Zhang-Shan-Forcing-Targeted-Lte-Cellphone-Into-Unsafe-Network.pdf

https://arstechnica.com/information-technology/2015/10/low-cost-imsi-catcher-for-4glte-networks-track-phones-precise-locations/

http://openlte.sourceforge.net/

http://www.softwareradiosystems.com/tag/srslte/
https://github.com/srsLTE