Transfer old number to micro sim?

I got a iphone 4s with a micro sim, i want transfer my old number and detail from big sim card on to the new micro sim. How do i do that? I'm on t-mobile in UK

Ring t-mobile, and tell them the new micro-SIM ID number
and your phone number will just move to it - might take up to an hour but usually a few mins

The mico sim usually has to have registerd and got itself a (temporary) number for itself, but talk to t-mo's help line, they will go do it.