It won't pair with my phone.

Try to reset your FitBit Alta, uninstall the App from your phone and start as fresh.

Steps for restarting FitBit is as follows :

1. Plug your charging cable into a USB port on your computer or any USB power adapter.
2. Insert the other end of the cable into the port on the back of your Fitbit Alta. Your Alta will begin charging.
3. Press the button on your charging cable three times within eight seconds, briefly pausing between presses. The button is on the end of the charging cable that is plugged into your computer.
4. Eight seconds after the first button press, you'll see the Fitbit logo on the screen. This means Alta is restarting.
5. After you see the logo you can unplug your tracker from the charging cable.