If it's a contact phone number ask sombody that you have called to text it to you, my brother does that alot ^_^
(Also if it's a contact number why does the app need it?, what is the app?)
If it's the fire tablets model/ part/serial/kernal number it should, hopefully, be in an "about device" file
On an android tablet it is found at the very bottom of the settings page.
Not entirely sure on an amazon system?