Autofill a integer field with a phone number based on the users name selected in the dropdown using ajax

I have created two custom fields in redmine.One a dropdown Field with users names and one more a integer input field where I manually type the phone-number of users selected in dropdown.My question is how to autofill a integer input field with the phone number based on the users selected from dropdown.Users Phone number will be stored in database.This phone number details will be entered when we create a new user in Redmine People Plugin. How to do this using ajax?If there is other way please tel me Thank you

PFA of screenshot.[![this is [![][1]][1]][1]][2]

[1]: http://ift.tt/20li2Ji - Custom fields with dropdown(Raised BY) and integer input field(Client ph-No)

[2]: http://ift.tt/1PLs9FJ - phone number field in redmine people plugin