It is possible that one of the towers in that area is down. Or the map may just be totally wrong. You can make T-Mobile aware of this by calling customer service and asking them to file a ticket for the area in question. They'll forward it on to the engineering team. Anyway, there is a disclaimer at the bottom of the Personal Coverage Check.
This map predicts and approximates our wireless coverage area outdoors, which may vary from location to location and may change without notice. It may include locations with limited or no coverage. Our maps do not guarantee service availability. Even within a coverage area, there are several factors, such as: network changes, traffic volume, service outages, technical limitations, signal strength, your equipment, terrain, structures, foliage, weather, and other conditions that may interfere with actual service, quality, and availability, including the ability to make, receive, and maintain calls. Multi-band (850/1900 MHz)
GSM/
GPRS handset required for access to 850 Roaming Coverage. Ask a T-Mobile sales representative or visit
tmobile/morecoverage for more information regarding handset requirements and T-Mobile 850 MHz coverage. 850 MHz roaming is offered through our roaming partners; some features (e.g. picture messaging, mobile to mobile) may not be available while roaming. Learn more >