I contacted T-mobile and they told me they would never call with details of better offers and if ever they called for any reason their phone number would be displayed.
Today I got a call from a man who said he was calling from T-mobile. the number displayed was 01952211670. He wanted to offer me an upgrade to a business account that would only cost £15 per month and give me tons more free calls, texts and also voice mail as well as a new handset. He said it didn't matter that my contract was still fairly new as it was with the same company. Still suspecting this might be a scam I suggested he e-mailed me the details. He told me his e-mail was not currently working but if I gave him an e-mail address he would try and do so later tonight.
After the call I phoned the number back and it was answered by an automatic answering service for Excel communications.
Should he e-mail me I shall report him and his company to Trading Standards but I doubt he will, somehow.
I have since checked the Excel-communications web site and they don't even deal with T-mobile in the first place.
See http://www.webologist.co.uk/2010/05/consumer-warning-excell-communications-and-orange-mobile.html for a review from someone who wasn't so lucky