if you call tmobile they will tell you that the phone doesnt support 3g , which it does its the network that doesnt fully support the phones, i had that same issue with the network here in the u.s , i even went to the store and they asked me what phone i had and they told me that the phone is not 3g. at the top left corner it showed 3g or sometimes 3.5 the phone is capable of it , not sure tmobile network is.
I apologize for the delay. In order to connect with the 3G . 4G network, a device needs to use both of the AWS 1700/2100 bands. Unfortunately, the Nokia 5800 does not support both bands required to connect to the T-Mobile 3G, 4G network. Altbhough T-Mobile does not guarantee full functionality of non T-Mobile phones, it sounds like you are able to connect to the EDGE network with no problem.
If you plan to use the picture messaging feature, you may need to input additional T-Mobile settings into your phone.
Hope this helps!