You can activate a T-Mobile prepaid phone either online or by calling the T-Mobile customer service number. You need the serial number found on the SIM card, the serial number for the phone and the activation code included with the product activation card.
When activating a phone online, open a web browser and go to the Prepaid section on the T-mobile web page. Setup options are offered for iPhones and all other cellphones. After selecting the appropriate phone type, enter the activation code and the serial numbers for both the SIM card and the phone. The serial number, or IMEI, for most phones can be obtained by dialing "*#06#" on the phone. To obtain the serial number for an iPhone go to Settings, General, and About. Serial numbers are also available on the original packaging.
The next page of the online activation asks for information to set up the account. You're required to enter personal information such as date of birth and email address. You can also create a PIN and opt to keep a current phone number. After completing the account setup, you must choose a plan in order to activate the phone. You are then able to add credit to the account and set up your billing details. There is an option to choose auto-pay based on your indicated top-up amount.