If you want to use an SDR instead of a dedicated 2G/3G/LTE modem, you would still need 2G/3G/LTE baseband firmware to teach your SDR how to access the cell network (if/when you wanted that). As mentioned by "pjc50", OsmocomBB (see https://osmocom.org/projects/baseband ) aims to do this for 2G, though it isn't quite ready yet. FreeCalypso (see https://www.freecalypso.org/software.html ) has a working 2G firmware, but is focused on very specific hardware. Note that any free/adaptable baseband firmware for 3G/LTE is still a long way off.
In the meantime, it's better to use a phone that gives you control over the cellular modem (if you still need the modem), as "confounded" suggested (with the Librem 5). Other phones similar to the Librem 5 are also specifically designed to ensure the modem is not doing anything unexpected, such as the Neo900: https://neo900.org/faq#privacy - "Unlike some other smartphones do, Neo900 won't share system RAM with the modem and system CPU will always have full control over the microphone signal sent to the modem. You can think of it as a USB dongle connected to the PC, with you in full control over the drivers, with a virtual LED to show any modem activity."
Regardless of which of these you choose, you can already separate your phone number from your SIM card with projects like https://jmp.chat/ so you don't need to use a cellular modem just to send/receive SMS/MMS and calls if you happen to be around wifi already. And you get other nice features like voicemail auto-transcribed and delivered as text by default, too (see https://jmp.chat/#voicemail for details).
So I'd start by using https://jmp.chat/ with your existing phone and phone number, then switch to new phones/modems as the modem separation gets better.
In the meantime, it's better to use a phone that gives you control over the cellular modem (if you still need the modem), as "confounded" suggested (with the Librem 5). Other phones similar to the Librem 5 are also specifically designed to ensure the modem is not doing anything unexpected, such as the Neo900: https://neo900.org/faq#privacy - "Unlike some other smartphones do, Neo900 won't share system RAM with the modem and system CPU will always have full control over the microphone signal sent to the modem. You can think of it as a USB dongle connected to the PC, with you in full control over the drivers, with a virtual LED to show any modem activity."
Regardless of which of these you choose, you can already separate your phone number from your SIM card with projects like https://jmp.chat/ so you don't need to use a cellular modem just to send/receive SMS/MMS and calls if you happen to be around wifi already. And you get other nice features like voicemail auto-transcribed and delivered as text by default, too (see https://jmp.chat/#voicemail for details).
So I'd start by using https://jmp.chat/ with your existing phone and phone number, then switch to new phones/modems as the modem separation gets better.