You can assign internal number to a virtual fax (by default it's 901).
PBX users can perform Call transfer to this internal number.
And then your peer will be forwarded to fax.
After fax is transmitted, callers returns to voice menu.
There he can, for example, wait for connection with operator and ask if fax has been accepted without errors.
Configuring instruction is located in section Configuring receiving fax through transfer to internal extension.