I have a fax connected to the hub; that number is associated with the hub (it's only used for faxing).
I have another line connected to and associated with a scout. No problem with that line (used for voice).
I can receive a fax with no problem at all.
However, I'm not sending faxes. The receiving fax (non-ooma) gives either a Comm Error 421 or 351.
I tried the *99 thing but that didn't seem to change anything.
Note the comma, which will add a pause. If you can't change the prefix, you'll need to enter it when you're entering the number to fax.
You may also need to adjust the baud rate of your fax to a lower value like 9600.