T-Mobile Cell Phone Using

I am a newbie, and today I have a question. My parent opened T-Mobile plan for near 1 year, and they have 2 cell phone when they purchased the plan. Otherwise, after 6 month they add me to their plan, and they have to pay 9.99 addition. But right now I told my Mom change her cell phone to a new G1 and I will exchange my cell phone to her. I like G1 but I have to wait like 1 year to change a new phone, also my parent can change it. My mother agreed that, but I worry about the SIM card. I don't know when we exchange the phone, and she uses my cell phone with her number (her SIM card simple), and I use a G1 with my SIM. We just exchange the phone, not the number. All of our cell phone has myFaves function, and when I try to exchange the SIM to my father Blackberry phone, nothing happens, but it just a sync for myFaves. So does T-Mobile accept that or they will not allow but they don't know what phone I use, or they will suspense our plan? Can someone answer me? I will appreciate that.

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You can switch SIM cards to any of the phones you want. MYFaves will sync up with that SIM card, even if it doesn't show up on the phone, it will still be connected to the number connected to the SIM card in the phone.

Now if you put your SIM card into your Dad's Blackberry, his phone becomes a dumb phone, and you won't be able to use his BB data services.