Many network operators have a single number (such as 901 or 123) to get to voicemail. With T-Mobile it's not so simple. We understand that the retrieval number seems to be in the format:
+44062 (then mobile number without the first zero). We've also heard of the prefix +44065 being used.
Two other useful snippets:
Dial 222 to get your voicemail information sent to your phone
Dialling *#67# should tell you what your voicemail divert number is