Failed fax messages typically occur when the outgoing provider's fax number is incorrect in the Other Organization dictionary. Anytime a fax fails to send for any reason, the user will get a failed fax message to let them know the fax did not go through.
The fax numbers for outside providers or clinics are stored in Super Admin -> Other Organizations - Person Search. You will need to confirm the fax numbers for the providers that you are receiving the failed messages for and update them in the BDH Other Organizations.
If you find that a fax number is correct in the Other Organizations dictionary, then you may need find the failed fax message in Messaging -> Faxes - Sent and click the resend button.