You can purchase multiple fax numbers to use in Salesforce. When you send a fax there is the Organization Fax Number you can select which fax number you want to send the fax from. Under A5 Fax Configuration you can set the default number to use for all users. You can also create a custom button/link to Send Fax that can pass the fax number you want to send from dynamically based on the user. This would be with a formula that would look at the current user or field to apply that logic. You cannot specifically assign fax numbers to users in settings but can be configured based on permissions. You can also apply sharing rules to Sent Faxes/Received Faxes records based on the fax number sent from or received to to share those records with the right users/roles/profiles.