To handle calls for multiple people or identities add lines for those users by clicking on the Add Line link. If your users are not managed by this PBX, click on Add External Line.
A common and convenient alternative to adding multiple lines to a phone, is to create a hunt group that will send calls to a list of users. For example: if Joe in sales is already assigned to the first line of this phone, instead of adding a second user called "sales" to this phone, create a hunt group called sales and add Joe to that group.