do you want this for your end user or the support-team?
It was to be on the end user form when they are raising an incident.
The form would show the users current phone number and have another field where they could enter an additional phone number incase they were away from their normal desk
The best method is to create a new user relationship on the process object and name this 'Self Service User'. You can then auto populate this with the current user and as such display other information from their user record. This is described here:
Hope that helps?