I moved to T-Mobile from Orange early this year when the half price offers were on. I got a 200minute/Web n' walk package on a Vario replacing my Orange smartphone. Not only that, but I bought a number of them for my business as well, replacing Blackberrys. And that was before push-email now available as well!
Yes, you can move your number at any time from another provider.
However, when I needed a second personal phone recently, I actually bought an Orange contract on a Nokia 6233 for they were doing a real special of 650 minutes an 1000 texts for ?30 per month.
That means I often carry bothe devices at the same time and this is where I advise a little caution. Almost invariably, in my area (Milton Keynes/Northampton. West Coast line to London) the reception on my T-Mobile phone is poorer than on Orange, except, curiously enough, in Olney. The train is particularly bad on T-Mobile, but not on Orange. (I have tried other, unlocked phones to see if it is the carrier or the handset).
So do give yourself the escape of 7 days' trial. After all, there is no point in a cheap package that means you can't make or receive calls often enough.