Redmine

I have installed Redmine People plugin. I entered the details of the user including the phone number. Also created the custom field named "Phone Number"(integer type) in Redmine. I have one more custom field named "Raised By" which is a dropdown List containg the users name. My requirement is , whenever I select the users name from "Raised By" field ,the custom field named "Phone number " must be autofilled with the users phone number which is provided in the People plugin.
Please tel how to solve this