T-mobile - iPhone and Blackberry question!?

[Aliisonnllx]

I have an iPhone on contract and I have just bought a Blackberry, I would like to use my iPhone sim card in my Blackberry to keep the same number. I have bought the sim converter to have it in my Blackberry. I want to use Blackberry services, but when I text t-mobile it says this function is not avilable on your price-plan. Has anybody else had this problem??

Please let me know...