Unlocking a cell phone

I am very proud of myself for unlocking an old ATT cellphone from so I can use with any sim card I want from any provider I want. I bought an unlock code online for $7.99 and then went to a T Mobile booth at the mall. I bought a "pay as you go" sim card for $20. When I inserted the T Mobile sim card, my ATT phone told me it was an invalid card. I inputted the unlock code and it is now unlocked. So now I also have a new cell phone number which I could consider my personal number.