Xfinity.com/activate mobile is a webpage where you can activate your Xfinity Mobile SIM card and start using your Xfinity Mobile service. To activate your SIM card, you will need to enter your Xfinity Mobile account credentials and a unique activation code that can be found on your SIM card. Once your SIM card is activated, you will be able to make calls, send texts, and use data on your Xfinity Mobile device.