Can T Mobile sim card be used in ATT phone and vice versa?

[m***@privacy.net]

Since T Mobile and ATT are both GSM based isn't it
possible to use phones and SIM cards between BOTH
systems?

[GeekBoy]

Post by m***@privacy.net
Since T Mobile and ATT are both GSM based isn't it
possible to use phones and SIM cards between BOTH
systems?

Only if the phones are not locked