Starting in August 2016, SSA (my Social Security) is adding a multi-factor authentication and will ask you to add your text-enabled cell phone number. If you do not have a text-enabled cell phone or you do not wish to provide your cell phone number, you will not be able to access your my Social Security account.
Many people who use the online services of the SSA do not have cell phones or do not use text messaging, especially older clients. They do however have EMAIL which is another method of multi-factor auth. In addition to text messaging and email SSA could also use voice delivery of the code to the users home phone on record.
Allow people to choose the method which works best for them instead of dictating what they use and cutting them off if they don't comply.