Have you got the correct T-Mobile APN settings in your Mobile network settings
Menu
Settings
Wireless & Networks.
Mobile Networks
Access Point Names
T-Mobile UK
T-Mobile Settings:
Internet
Name: T-Mobile UK
APN: general.t-mobile.uk
Proxy: 149.254.201.132
Port: 8080
Username: t-mobile
MMS
MMSC:
http://mmsc.t-mobile.co.uk:8002
MMS Proxy: 149.254.201.135
MMS Port: 8080
MCC: 234
MNC: 30
Authentication: Normal (PAP)
APN Type: Internet + mms
SMS
Service Centre No. +447958879879