Be warned im pretty dumb when it comes to phones....

Ive had the same tmobile phone for years thing finally broke. So i went to target to get a new tmoible phone for cheap, i dont need anything fancy. They told me they only sold simple mobile now and that putting my tmobile SIM in that phone would work just fine. So i get home and pop it in and now it says...

"SIM card info - The SIM card does not match the device, please change the SIM card which the operator required"

Then the main screen says "Invalid SIM card - Device is locked!"

Do i need a different kind of SIM card or did the Target people bamboozle me and this phone really doesnt work with Tmo. I'm thinking i need to return it and get my 30 bucks back!