HOW TO UPDATE USER:
Go to Configuration
VOIP Setting Tab
Find the user and click Edit
Edit the rule Add user you want and remove the old one
Rules & Routing make sure it has the new user in the Recipient section
Go to the User management, find user and check the  "Enable VOIP" box submit and the phone number will pull in.

HOW TO ADD A NEW USER NUMBER:
Go to VOIP Setting Tab
Add Rule
Find a number assigned
Select
Type-move to inbound/outbound
Add User
Update the Rules & Routing 
Go to the User management, find user and check the  "Enable VOIP" box submit and the phone number will pull in.