If you are plugging in only the fax machine to your ooma, ooma would be the walljack for these DSL filters. I've had success with these filters inline when calling machines such as fax and dish central.
I am sorry to hear about the troubles. Let me help you with this problem. About the *99, it really depends on the fax machine if it would adopt the protocol where it's connected. Not all fax machines are able to work when the fax mode is enabled so there will be times that you would really need to dial *99.
With regard to incoming fax, I noticed that the second number was recently added. If this was not yet done, please unplug your Ooma device from the power then plug it back in after two minutes then test if you will be able to receive the fax messages.
If I interpret the above correctly, it means that fax mode may work for outgoing faxes or it may not. Apparently *99 does something extra over fax mode. Also apparently after enabling fax mode you need to restart the Ooma device.
Note this is from Ooma email support, so I'd take it with a grain of salt as I've found that they don't always have the correct information.