Stealing my own mobile phone number

When in the US, I’ll usually avoid roaming fees by using a T-Mobile SIM card and a Boston number. Due to a recent phone upgrade, I had to move to a different SIM card form factor.

Imagine my surprise when the interaction at the T-Mobile shop in Berkeley today went, roughly, like this: “What’s your number” — ” 857 …” – “Thomas?” – “yes” – “Hold on.”

I paid for the new SIM card, in cash. I put it into the recently-acquired phone. It worked. I walked out of the shop. At no point did I have to prove ownership of a SIM card that belonged to that phone number. And at no point did I have to produce any credentials.

Now, I’m suspecting that some of this might be related to me lacking a US street address — I’m just traveling here. But even if they were to ask me about an address: Just knowing somebody’s phone number and address, and nodding convincingly when asked whether I’m their first name, doesn’t strike me as a useful way to check that I actually am the owner of that number.

Anybody else see a problem here?