While T-Mobile has done an amazing job of improving its network coverage, it is simply impossible to provide full bars in all locations at all times. It is up to the user to take some reasonable steps to improve their coverage in locations where the signal is weak. Thankfully, these steps do not require too much hassle and may resolve the issue outright.
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Power your phone off and then back on. Especially while driving or when moving to a new location, issues with weak signal may be due your phone not switching over to a local tower. Power-cycling prompts your phone to reacquire local signal and re-register on the network.Once the device is back on, see if your signal has improved.
Manually select a network. If power cycling did not result in stronger signal, you may need to manually force your phone to search for and register on a local network. Generally this is done by pressing "Menu," "Settings," "Network Setup," and "Manual Network Selection." Select T-Mobile from the list of networks that appears and hit "Register." After it registers, check your signal again to see if it has improved.
Purchase a cell phone repeater. If the above steps did not result in boosted reception, it is possible that the area you are in just does not get good signal. If you are in a location where signal is good outdoors but not inside, a cell phone repeater can pick up the signal and amplify it throughout the building. While repeaters cost anywhere between $200 and $600, this may be the best solution short of T-Mobile installing a new tower.