Or better yet: Take a cheap, low power SoC like the #Pi0W and add a cheap Baseband like a #SIM7600G to it and some simple GUI that does the AT commands to do calling, SMS & data connections...
https://www.waveshare.com/sim7600g-h-4g-hat-b.htm
SIM7600G-H 4G HAT (B) For Raspberry Pi, LTE Cat-4 4G / 3G / 2G Support, GNSS Positioning, Global Band
TelH90