Here in Vegas my bb switches like that alot .
But you will not get charged for roaming . Tmo and ATT have reciprocal agreements and use eachothers towers to boost coverage .
If you are on ATT network with Tmo as your provider , you may not be able to access all of TMO's services , such as internet connectivity and in some cases voicemail . But it will switch back to TMO's carrier as soon as it gets a strong enough signal . If you are on a call when it happens , you probably will loose the call :(