You can put a SIM card in some older thinkpad laptops with that upgrade option. Some thinkpads have the slot for a SIM card but not the internal components to use it. So make sure to do some research if that sounds promising.

There are VOIP phone line services like JMP that give you a number and let you use your computer as a phone. I haven't tried JMP but it always seemed cool and I respect that the developed software running JMP is open source.. The line cost 5$ a month.

Skype also has a similar phone line service. Its not open source like JMP and is part of Microsoft. Usually thats cause for concern for FOSS nuts, but in this context its not a bad thing in some ways. Skype is two decade old mature software with enough financial backing from big M to have real tech support and a dev team to patch bugs, in theory. So probably less headaches getting it running right which is important if you want to seriously treat as a phone line. I think Skype price depends on payment plan and where you live, so not sure on exact cost.