In order to start making phone calls through your cell phone, you will need to verify your mobile number. To do this, simply click on Settings:

Then, under General, enter your mobile number:

Once you have entered your mobile number, click on Update and you will automatically get a text message saying the following:

Reply to that message with a Y and you will receive a confirmation text saying that your phone has been confirmed:

And you are now all set to start making phone calls via cell phone.
If you want to learn how to call parents directly from the student card click here.