Annoying Razr v3 issue

Hi. I am new here but I NEEEEEEEEEEEED help! Please! I purchased a brand new unlocked Razr v3 from ebay. I was assured itwould work with at&t. It does, but the seller failed to tell me that the phone was tmobile branded....and I've read that when you use an at&t SIM card in an unlocked T-mobile phone, you mgiht run into some issues...well I did....calls and texting work awesome...but every time I turn the phone on I get a text message from this number-1121611611 that says:
" 435 error invalid number. please resend using a 10 digit mobile number or valid short code. 06050415ccc00160d99b568d58b960c5a3564cdcb4720000" This text message is costing me and I can't let this continue. It will eventually add up to a lot of charges! Can you please help me with this? Why is this happening?

PLEASE HELP ME! This is annoying and is costing me money. And I've already heard the "just don't turn it off" bit...well that's ot practical. Thanks in advance!

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Reply to the text "STOP" If they continue you'll have to contact AT&T and they should credit you for any texts you receive after that.

Nobody knows how to fix this problem? I have the same phone, same T-Mobile branded and using att prepaid. I've googled and googled and I'm about to go postal!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!1

thanks mon frére