Most websites have blocked VoIP numbers so you won't be able to use them for 2fa.
I’ve come across this as well, and having worked in VoIP the common rationale is fraud prevention, which is a real problem that does exist-but I think it’s a rationale that has been quite misapplied or perhaps better to say, inappropriately applied as a blocker for using VoIP numbers for 2FA.
In my experiences the vast majority of websites allow it -- a select few disallow. Although that group will expand. One trick I've found is to register the 2FA with a "real" number, and then port that number to a voip service. I never use a real cell phone number for anything.
I've eyeballed family plans a couple times, in some cases it's just a minimum of 2 phones sharing a larger data pool. I've toyed with the idea of putting the 2nd SIM in my old phone and just having it around for... whatever, as long as it's not consuming data.