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Debugging Common Issues with Fax

This table outlines several of the common error messages, their causes, and potential workarounds that users encounter when sending Faxes.

Error Description Likely Cause Workaround
Connection Failed There was no receiver on the other end of the call to connect to. The destination is not configured to receive facsimile transactions. 1. Retry
2. Contact the owner of the destination number to confirm compatibility and connectivity.
Document generation error There was an error while generating the document. This could be due to the file being deleted before sending. Internal Service Error 1. Retry
2. If the issue persists, contact Twilio support
Failed to train with any of the compatible modems A remote fax machine was detected, but the sending and receiving modems could not establish communication. Connectivity issues 1. Retry
2. Contact Twilio Support
Fax transmission not established We could not detect a remote fax machine. This could be due to there being no fax machine on the receiving end or a lack of call clarity. Connectivity issues, Carrier issues, T.38 Incompatibility issue 1. Retry
2. Determine whether remote side was configured for T.38 only. Twilio only supports Faxes sent using the T.30 protocol.
Log entry lost The call started, but the log entry for the call was lost/incomplete. In practice, this is rare and may indicate a system problem. Internal Service Error 1. Retry
2. Contact Twilio Support
No response after sending a page The remote fax machine did not acknowledge that it received a page of the fax. Depending on the remote machine's behavior, it may still have printed the page and any preceding pages. Connectivity issues 1. Retry
2. Contact destination to ensure delivery
Received no response to DCS or TCF The bulk fax service could not successfully determine the remote machine's fax capabilities. T.38 Incompatibility issue Determine whether remote side was configured for T.38 only. Twilio only supports Faxes sent using the T.30 protocol.
The call dropped prematurely The call dropped due to a non-fax transmission error. It is likely that the receiver hung up. Inbound to Twilio: The destination is a voice number or not set up to receive faxes.
Outbound: The destination is a voice number, or the destination returns ringing for so long that the transmission times out before the handshake can be established.
Retry
Note: Twilio Fax only supports the transmission of PDFs to Fax-enabled destinations.
The HDLC carrier did not stop in a timely manner The fax service initiated a fax transmission with the receiver, but there was a synchronization (timing) error that could not be resolved. Likely low-quality routes 1. Retry
2. If network conditions persist, contact Twilio Support so we can contact our carrier partner for additional insight.
Timed out waiting for initial communication A call was established with the receiver, and the bulk fax service attempted to establish a fax session. However, there was no fax response from the receiver. This could be due to there being no fax machine on the receiving end or a lack of call clarity. No remote fax machine or poor quality routes 1. Retry
2. If network conditions persist, contact Twilio Support so we can contact our carrier partner for additional insight.
Unexpected DCN while waiting for DCS or DIS The remote fax machine unexpectedly sent a disconnect message when the fax service asked for the remote fax machine's fax capabilities. T.38 Incompatibility issue Determine whether remote side was configured for T.38 only. The protocol is not designed to recover from these situations; all that can be done is fail.
Disconnected after permitted retries The fax service attempted to send the same message multiple times unsuccessfully. this may be due to a call clarity issue. Poor network conditions 1. Retry
2. If network conditions persist, contact Twilio Support so we can contact our carrier partner for additional insight.
Far end cannot receive at the resolution of the image The remote fax machine does not support receiving faxes sent at the resolution sent by our service. Quality is too high Try lowering quality. (Resolution is known as "quality" in Programmable Fax.
Received a DCN from remote after sending a page The remote fax machine responded with a disconnect message after a page was sent successfully. Depending on the remote machine's behavior, it may have still printed the sent page and any preceding pages. Poor network conditions 1. Retry
2. If network conditions persist, contact Twilio Support so we can contact our carrier partner for additional insight.
Unexpected message received The fax service received a message that it did not expect given the current context. Protocol failure Determine whether the expected number of pages matches the number of successfully transmitted pages. Failure may have been in tearing down the call.
Unexpected DCM after EOM or MPS sequence The remote fax machine disconnected unexpectedly after receiving a page of a multi-page fax. Depending on the remote machine's behavior, it may have still printed the sent page and any preceding pages. Poor network conditions 1. Retry
2. If network conditions persist, contact Twilio Support so we can contact our carrier partner for additional insight.
Document loading error The fax service attempted to generate the message to send, but a document was missing. Internal service error 1. Retry
2. If issue persists, contact Twilio support.
Invalid ECM response received from receiver The fax service received an invalid error correction message from the remote fax machine. T.38 Incompability issue 1. Retry
2. If failures only occur with one number, there's a chance that the remote fax machine is incompatible with our infrastructure. Twilio only supports Faxes sent using the T.30 protocol.
Unexpected DCN after RR/RNR sequence The remote fax machine disconnected unexpectedly after indicating that it was not ready to initiate a fax session. Unexpected failure 1. Retry
2. If failures only occur with one number, there's a chance that the remote fax machine is incompatible with our infrastructure. Twilio only supports Faxes sent using the T.30 protocol.
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