Technically speaking, a Cingular phone can be used with T-Mobile, as you only need to switch the SIM card. However, the phone may be locked to Cingular, in which you need to unlock it to be used with another GSM network.
You can't get a T Mobile service on a phone that's not T Mobile unless you get it unlocked. Once the phone is unlocked, you can use your T Mobile SIM card on it or any other GSM SIM cards worldwide.
A prepaid plan can usually be carried over to an existing phone or new phone of the carrier. T-Mobile phones can be switched over easily by yourself or a professional.
If you are trying to use the T-Mobile phone with your Cingular SIM, and maintain your Cingular service, you will have to call T-Mobile to get a Subsidy Unlock code, and unlock the phone to be used...
So you are saying that you want to use a Cingular phone with T-Mobile? Or the other way around? Well yes you can. You need to have the phone unlocked. You can have this done for free by calling the...