From Social Security:
The Social Security Administration (SSA) has added an extra layer of security for our customers when they interact with us online using the my Social Security suite of services. my Social Security account holders are required to use their cell phone, in addition to their username and password, as an additional authentication factor during online registration and every sign in.
We implemented multifactor authentication (MFA) to comply with Executive Order 13681, which requires federal agencies to provide more secure authentication for their online services. We are committed to using the best technologies and standards available to protect our customers’ data. MFA is just one of the ways we ensure the safety and security of the resources entrusted to us. Since we launched my Social Security in May 2012, we have provided this added security of MFA as an option to our customers.Now, all new and current my Social Security account holders will need to provide a cell phone number able to receive text messages. People will not be able to access their personal my Social Security account if they do not have a cell phone or do not wish to provide the cell phone number. We expect to provide additional options in the future, dependent upon requirements of national guidelines currently being revised.
Don't anybody tell Social Security that it's possible to get text messages without a cell phone.