activated my old iPhone 4 on Tmobile and it works but it somehow connected to my iphone 5 on att. every time get text on one other gets it also. Help stop this please

I activated my old iphone4 on Tmobile.  ATT unlocked it and i got a new sim card from Tmobile.

It works but it is somehow connected to my iphone 5 on att. 

Every time i get a text on one the other gets it also. 

Help stop this please