To change an employee's phone number, access the users tab and click the user you wish to edit. Type the new number and click anywhere outside the phone number box. A bouncing ball will appear, indicating that the number is being saved.
If the phone number is not being saved, it could be related to two reasons:
That the phone number already exists and it is part of another employee's profile.
That the phone number belongs to a land line.
Every user should be attached to a unique mobile phone number which will prevent any external person to log in your app.