Some companies maintain secret lists of phone numbers which they prohibit for new signups. If you run into this problem, you can:
- try again (sometimes this solves the issue)
- contact the company, letting them know of the issue
- use a different form of 2FA, like OTP (since SMS 2FA is neither private, nor is it all that secure
- find an alternative to that company's services
- use a service specifically tailored to SMS 2FA instead, like https://crypton.sh/ or https://smsprivacy.org/ (their numbers are not on these block lists, but they lack certain other features of a JMP phone number)