Can anyone on TMobile explain the Digital Roaming policy to me?

[Scarpozzi]

I'm just wondering about what TMobile charges for digital roaming. The website makes a statement that sounds like there are no additional roaming charges by the network you roam on, but it's a confusing statement. Has anyone with TMobile been charged a lot to roam? (example of areas with digital roam are Knoxville, TN and most of North Carolina)