An internal extension could be assigned to Virtual fax.
By default it's 901.
In case if caller gets into voice menu after call to company's number, then when dialing this internal number caller gets to virtual fax.
After fax is transmitted, the caller returns to voice menu.
There he could, for example, wait for connection with operator and ask if fax has been received without issues.
Configuring instruction is located in section Configuring receiving fax through voice menu.