Common reasons faxes fail include:
Sending to an out of service or non-working number
Sending to a regular phone number
Sending to an email address or Direct messaging address
Sending to a recipient who has added their number to the National Do Not Call List
Sending to a busy line
The recipient fax machine being out of paper
Noise or interference on the line, caused by things like construction or weather events
Carrier or provider-side routing issues
Carrier or provider-side outages
The system retries faxes a minimum of 5 times before the status updates to Failed.
Understanding Fax Error Codes
When a fax fails, hover over the 'Failed' status in Release to see the error code as shown below:
Use the table below to identify what happened and what to do next:
Error Code | Portal Message | What It Means & Next Step |
OT3936 | Unable to connect. Possible voice answer. Please verify recipient fax number. | A person answered instead of a fax machine. Confirm the number is a dedicated fax line and try again. |
OT3935 | There was no answer. Please verify recipient fax number. | The fax number did not respond. Confirm the number is correct and that the recipient's fax machine is on and available. |
OT3931 | The recipient fax line is busy. The system has attempted the maximum number of retries. Please contact the recipient. | The recipient's line was busy across multiple attempts. Try again later or contact the recipient directly. |
OT2013 | Recipient disconnected call before message delivery. | The connection dropped before the fax finished sending — likely a temporary issue on the recipient's end. Try resending. |
OT6600 | Partially Received Fax. Did not receive an expected end of transmission communication. | The fax was only partially delivered. The connection ended before the full document was received. Ask the recipient to confirm what arrived and resend if needed. |
OT6500 | Recipient is blocked. | The destination fax number is blocked — it may be invalid or flagged. Ask to speak with a member of the Kno2 Healthcare Network Services (HCNS) team if you believe this is an error. |
OT6503 | Recipient is blocked due to membership in a do not call list. | The recipient's number is on a do-not-contact list. Faxes cannot be sent to this number through your Kno2 Connected™ app. |
OT1901 | Fax Number is an invalid format. | The fax number is not formatted correctly. Check for extra characters, spaces, or missing digits and try again. |
OT-1 | No pages to deliver. Fax not sent. | No document was attached. Make sure a file is attached before submitting the fax. |
Verifying a Fax Number
Calling the number is the first troubleshooting step that should be taken before requesting assistance with a failure. If the line does not answer with a fax tone, fax troubleshooting cannot be completed.
A few indications the line is having issues include:
The call receiving a busy signal
The call ending before a connection is made
The call ringing without ever connecting
The call being answered with an "out of service" message
The call being answered by a reorder tone (fast busy signal) indicating technical issues.
The call being answered by a person, auto-attendant, or voicemail
Troubleshooting Failures to a Live Fax Line
If you call the number and hear a fax tone but continue to encounter consistent failures when attempting to send to it, an initial step you can try is changing your Caller ID number.
The Default number is common number for the outbound Caller ID to improve deliverability, but some recipients may block it in error. Changing the caller ID to reflect your fax number may resolve the issue.
If that does not resolve the issue and you need more information about a fax failure to a good number, please ask to speak with a member of the Kno2 Healthcare Network Services (HCNS) team and provide the URL of the failed fax.
To obtain the URL, open the failed fax in Release and copy the message URL from your browser's address bar.
If the URL is not available, provide the following information for two failed faxes for each failing number you wish to have reviewed:
Date (must be within the last 24 hours)
Time
To number
From number
Failure reason, if available
Need to resend a failed fax? Use Forward a Message to Resend It to send the content again as a new, separate fax transaction with its own delivery status (Sent, then Delivered or Failed). The original message remains in its Failed state.