For a government website like Social Security, the fallback is for the citizen to go to the local office instead of authenticating credentials remotely at home. Same situation with the IRS tax refund website. The ID.ME identity website used by the IRS rejected VOIP numbers. If a person wants their tax refund released without giving up their phone # to ID.ME, they need to drive to the IRS office and verify their identity in-person.
For the software-verify-phone#-with-SMS, there was no other option. If one theoretically didn't have a real mobile phone #, there was no way to buy that software.