The only information I've ever seen about this error is from the Blackberry Technical Solutions website. Here is a link to their article on the issue: Radio Path Unavailable
The battery and SIM card removal method has *always* fixed it for the folks I got on the phone with that issue. If you continue to have the problem then it does either sound like a network related issue in your area or an engineering issue with your account.
I would continue to call into T-Mobile and push until they submit a trouble ticket to get the engineers involved. At the very least, ask them to submit a service request for your home address. This would get the local engineers in your area to go out and check the cell towers and equipment.