I'm not sure what you mean by that. All authenticated users can use the send a fax tool, and when the inbound service is launched, you'll be able to have it send the faxes as pdfs to a list of supplied email addresses.
Sorry! I may not be using the right terminology.
Most Sonic.net accounts allow creation of multiple mailboxes @sonic.net, along with the firstname.lastname@example.org
When one connects to members.sonic.net, one can login with the primary_username ID and configure voice, create new mailboxes, manage the domain(s), and access Labs. If one connects to members.sonic.net and logs in with one of the additional mailbox IDs, the only options are "Account" and "Email" (see the attachment).
Is there any chance in the future that one could log into members.sonic.net with one of the additional mailbox IDs, find Labs on the menu, and send a fax? For example, in a family situation, where several family members have mailboxes as part of the account, the account owner may want to give fax sending permission to the family members, but may not want the kids to know the login password for the primary username.