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/Sessi... https://www.openairinterface.org/ https://media.defcon.org/DEF%20CON%2024/DEF%20CON%2024%20presentations/DEFCO... https://arstechnica.com/information-technology/2015/10/low-cost-imsi-catcher... http://openlte.sourceforge.net/ http://www.softwareradiosystems.com/tag/srslte/ https://github.com/srsLTE