It’s not like you have to give your SSN to a carrier to get a phone

Actually it is like that, if you are getting any kind of deal where you’re paying off the phone with your service plan and/or commit to a term contract. They use it to run a credit check on you. Most companies where you’re committing to a length of service do this. It happened to me when I was going to get some kind of cable or internet service one time, where you got x number of months free if you promised to keep the plan for two years. They asked for my SSN and I refused, so they wouldn’t complete the transaction. That’s how I found out about why they want your SSN.