T-Mobile are the popular network that has now merged with another popular network, Orange, to create Everything Everywhere (EE). Through their attempts to gain ground in the mobile phone manufacturing market over the years, T-Mobile have released some popular handsets such as the Vairy Touch range and Vivacity models. The company never really competed with the likes of Nokia or Samsung as a leading mobile phone brand, but the handsets still sold well enough to create a demand for them in the unlocking industry.
T-Mobile phones only require one unlock code to remove the restriction preventing you from using an alternative network. You should note however that if you want to use Orange or EE, the T-Mobile phone should already be able to do so. This was one of the results of the companies joining forces, with customers now able to switch between them freely. However, if you want to travel abroad or use a different local network, you will have to unlock it.