Faxes can fail for a variety of reasons, such as sending to a non-working or non-fax number, a recipient listing their number on the National Do Not Call List, a recipient line being continually busy or out of paper, noise or interference on the line, or provider-side routing issues. The system retries faxes a minimum of 5 times. If a released fax has a 'Failed' status, you can hover over the status for more error details.
Additionally, fax failures can also occur when attempting to send to non-fax line recipients, such as Direct addresses, making the transmission process incompatible.
Understanding Fax Error Codes
When a fax fails, hover over the 'Failed' status in Release to see the error code. 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. Kno2 has attempted 4 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
You can verify that the number is a working fax line by dialing it and listening for a fax tone. 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.
Attempt a test fax or call to confirm whether the line has issues such as noise or interference, which are technical in nature and can impede successful connections.
If you encounter consistent failures to a 'good' fax number, try 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.
Best Practices for Avoiding Fax Failures
Verify that all recipient contact information, including fax numbers , is accurate.
Call the fax line to ensure it answers with a fax tone.
Perform test faxes, especially when you anticipate potential issues with the recipient line.
Maintain open communication with recipients to resolve any contact-related inconsistencies.
Utilize Kno2® troubleshooting resources for additional guidance when required.
If 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 following details:
The URL of the failed fax.
To obtain this, open the failed fax in Release and copy the message URL from the top of the browser to share with us.
URLs look similar to this:
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 retry a failed fax to a good fax number? You can Forward a Sent Message to send your original attachment again!