I am not sure if someone posted it already, but as O2 started to charge for 07744/07755, T-Mobile did the opposite - they included it in your monthly allowance.
TBH, I never saw any official statement on this, but people on MSE swear that this is true. I tried it myself and it worked.
Someone had mentioned that it doesn't work for U-Fix customers though. I am on FlexT Web-n-Walk...
Disclaimer - if you are not sure about 07744/07755, then call T-Mobile and talk to their customer support. I am not liable for any charges you may incur, etc.:)