Curious. Some things to consider. MM and the modem see the network, meaning that part of the way works - antenna, network etc., so mostly not hardware issue, unless you got that unexpected short too. Any modem hardware issue would need a replacement modem. If it’s a network issue (that the 2G previously "masked from you), you’d need an alternative modem or maybe you could get some results if you go to a different area and try some other network than Mint (if there is any that works [without 2G] and is available). A shorted modem that has a visible mark and some blue smoke would be the easiest to diagnose but there can be something wrong even without any visible marks. Of these two alternatives I think the timing in relation to 2G seems too much of a coincidence. From there, towards the other end of the links in the chain of technology, the third alternative to look into is the software, if/what has changed. [Edit to add: LTE/data seems to often work even if “phoneline” doesn’t (did it here too?). A workaround could be to use SIP calls via data.]
In a bigger picture: there should be a process to troubleshoot phone issues. Not just with Librem 5 but all linux phones. There are so many links in the tech stack that need to work but it’s bit much to ask that a single user could fathom them all. Yet, this is and should be logical and there should be some straight forward methods to create a flowchard to rule out stuff and point to right direction. There should be a list of commands that gather all relevant information from all the different logs and sources - an presents them in a readable way (GUI and terminal/txt). I’m pretty sure all the different linux HW/SW/OS platforms could benefit from this. Seem kinda amazing that there isn’t a community app/tool that tells user about the health of their telephone connection (from audio to calling to modem software to modem to network to ISP…) and maybe even offer some test call functions somehow.