Can I put my AT&T sim card in my T-Mobile Samsung phone?

I like my Samsung phone better than the LG Vu that I have now and am wondering if I can put in a code to use my sim card in the old phone.

Thanks!

HI yes after unlocking the old Samsung phone from t-mobile you can freely use it with AT&T sim ..You can easily unlock your phone using sites like MobileUnlockSolutions.com here they are rendering unlock code for all mobile models at reliable cost ..After unlocking your Samsung you can freely use any GSM sim in it .