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Faxing is still a critical business requirement for many industries. The faxes app provides a set of faxing related features to kazoo accounts and supports sending and receiving faxes directly from email or the API. The fax entities can be tied to a callflow to terminate incoming faxes without having to use a fax machine.

Receive Fax Callflow Element#

The original fax application feature added the ability to create a call flow element that can be attached to a phone number and used to terminate faxes, generating an email to the user with the received faxed document attached. This feature has been replaced by the faxbox feature set, but the receive fax call flow element is still supported currently as a legacy feature.


The faxbox acts as a virtual fax machine. The faxbox is used for both receiving and sending faxes. The faxbox is tied into the API and Email Notification systems and will send receipts or error notifications when a fax is sent or fails to send, send notifications when a fax is received and history and content of sent and received faxes can be retrieved via the faxes API. Faxboxes can be added to kazoo call flows just like receive fax, but unlike receive fax, multiple email addresses can be added to the notification list. Unlike receive fax, faxboxes can be used to send faxes, which can either be uploaded via the faxes API directly, or submitted by sending an email to the faxbox smtp_email_address or custom_smtp_email_address or the account realm. The faxbox supports multiple file formats for sending faxes, most commonly, pdfand tiff, but OpenXML and other OpenOffice compatible documents can be configured as well.

The faxes API also provide an email inbox/sent items type interface for accessing sent and received faxes and debugging failed fax transmission attempts. Checkout the faxbox documentation for more details on how the faxbox works.

Api Fax#

In addition to sending faxes via email to fax, the crossbar application provides an API that can be used to deliver faxes using two methods, url and multipart.


If the url method is used, a url to access the document must be provided in the request body using a url that is reachable by kazoo.


If multipart method is used, the fax document is attached as part of a multipart body, which also contains a JSON object which provides the from and to numbers for the fax.

Sending Faxes#

Two methods are provided for sending faxes, api fax and faxbox. Faxbox can use either google cloud printer, or the fax_smtp server to handle requests to send faxes. Api fax uses the crossbar API to provide the fax content. When the request is received via any method, the requested fax document is processed, a document is created in the faxes database, and the document is set to a pending status putting it into queue per account by fax_monitor, by spawning an instance of fax_jobs using the unique name fax_outbound_{account_id}.

The fax jobs module provides a queuing and rate limiting mechanism per account, defaulting to max_outbound at 10 active jobs per account. For the permitted number of jobs in queue, fax_jobs spawns an instance of fax_worker under the global name fax_outbound_{destination phone number} or if serialize_outbound_number is disabled fax_outbound_{random_uuid}. By default only one fax job per outbound number can be executed system wide, if serialize_outbound_number is false, only the per-account limitation is used to serialize jobs and multiple outbound faxes to the same number can be initiated.

Fax worker fetches a fax compatible file from the queued fax job, the file is then written to the configured file_cache_path, this file is served via the fax_file_proxy module which provides an HTTP file server accessible from the FreeSWITCH servers in the cluster, and transmitted using a FreeSWITCH originate to the destination phone number. Upon receiving the originate request, the FreeSWITCH fetches the fax document from the fax_file_proxy and attempts to transmit it using the T.30 or T.38 protocol based on configuration and negotiation with the far end.

If fax transmission fails, and the system_config.fax document has reschedule rules configured, these are applied, based on the type of failure. For example, a reschedule rule can be added for user-busy condition that will result in a re-transmission after a configured retry-after period. If no reschedule rules are applied the transmission will be re-attempted until the configured number of retries on the faxbox is exhausted.

When all retries are exhausted, the fax will be put into state failed and an email notification will be sent indicating the cause of the error. If the fax transmission is successful, a receipt will be sent to the user and/or any email addresses specified in the faxbox outbound notification list.

Receiving Faxes#

For receiving faxes, either the receive_fax or faxbox callflow elements must be attached to a callflow. When an incoming call is placed to the callflow's phone number, FreeSWITCH receives the call, passes this up to the call control layer via ecallmgr. The callflow is matched via the callflow app the fax reception is then handled via the fax_request module. The t38 configuration and the filename is provided to FreeSWITCH to use to cache the file and then the fax handshake and transmission is handled via the FreeSWITCH.

When the transmission succeeds, the file is put in the directory defined in the fax_file_path in system_config.ecallmgr. A document is created in the db then fax_request instructs the FreeSWITCH to upload the document to the database, using the naming scheme received_fax-<fax-id>.tiff. Once the fax content is uploaded to the db, a notification is generated with the message attached in the format defined in fax.attachment_format defined in system_config.kazoo_convert (tiff or pdf).

When the transmission fails, an error notification is generated indicating the cause of the failure.

Kazoo Fax Converter And Attachment Storage Handling And Configuration#

If a document is submitted for an outbound fax request in a format unsuitable for faxing (word or OpenXML document, pdf or tiff in a non-faxable format), the attachment will be converted to a fax compliant tiff. The conversions by default will occur whenever a fax document is received via the API or fax_smtp server.

The parameters store_fax_tiff, store_fax_pdf and store_fax_url are used to dictate if the results of the conversions should be stored in the database, or if conversions should take place on the fly when a file in one of these formats are required. By default kazoo will store the original document, a faxable tiff version of the document, and a pdf generated from the faxable version of the document.

If these settings are NOT enabled, kazoo will fetch the original document either via the original file or if a url document is specified, fetching the document from the url using HTTP. Kazoo will then execute any conversions required for the type of file requested (faxable tiff or pdf) and do those conversions each time the fax is requested. This is much slower, especially for the API, but does reduce the size of the database documents since only the original document or document url is stored in the database. If url fax documents are provided and store_fax_url is set to false, every time the file is requested, it will be downloaded from the url, converted to the format required and that document will be returned.

Configuration Parameters#

system_config.fax global settings

Key Description Type Default Required Support Level
allow_all_addresses_when_empty fax allow all addresses when empty boolean() false false
allowed_content_types.[] string() false
allowed_content_types fax allowed content types array(string()) ["application/pdf", "image/tiff", "{"prefix":"image"}", "{"prefix":"application/vnd.openxmlformats-officedocument."}", "{"prefix":"application/vnd.oasis.opendocument."}", "application/msword", "application/", "application/"] false
cloud_registration_pool_interval fax cloud registration pool interval integer() 5000 false
default_compare_field fax default compare field string() result_cause false
default_fax_extension fax default fax extension string() .tiff false
default_retry_count fax default retry count integer() 3 false
default_retry_period fax default retry period integer() 300 false
default_smtp_domain fax default smtp domain string() false
delete_empty_faxes fax delete empty faxes boolean() false false
denied_content_types fax denied content types array(object()) ["{"prefix":"image/"}"] false
endpoint_timeout fax endpoint timeout integer() 40 false
ensure_valid_caller_id fax ensure valid caller id boolean() true false
fax_file_path fax fax file path string() /tmp/ false
fax_settings fax fax settings object() {"override_fax_identity":true,"override_callee_number":false} false
file_cache_path fax file cache path string() /tmp/ false
ignore_early_media fax ignore early media boolean() false false
image_min_size fax image minimum size string() 700x10 false
image_size_cmd_format fax image size cmd format string() echo -n ```identify -format "%[fx:w]x%[fx:h]" ~s`` |false`
inbound_t38_default fax inbound t38 default string() true false
log_faxbox_errors fax log faxbox errors boolean() true false
max_outbound fax max outbound integer() 10 false
max_storage_retry fax maximum storage retry integer() 5 false
port fax port integer() 30950 false
report_anonymous_system_errors fax report anonymous system errors boolean() false false
report_faxbox_system_errors fax report faxbox system errors boolean() true false
report_smtp_errors Report SMTP-related errors via notifications boolean() true false
reschedule fax reschedule object() {} false
serialize_outbound_numbers Serialize fax transmissions by outbound number globally boolean() true false
smtp_max_msg_size fax smtp maximum msg size integer() 10485670 false
smtp_port fax smtp port integer() 19025 false
smtp_sessions fax smtp sessions integer() 50 false
store_fax_pdf store the post processed fax document boolean() true false
store_fax_tiff store a pdf copy of the post processed fax document boolean() true false
store_url_document store the document url result in the database boolean() true false
wait_for_fax_timeout_ms fax wait for fax timeout in milliseconds integer() 3600000 false
workers fax workers integer() 50 false
xmpp_interval fax xmpp interval integer() 600000 false

Useful Sup Commands#

To see the pending faxes currently in queue:

sup fax_maintenance pending_jobs

To see the active faxes:

sup fax_maintenance active_jobs

To see the jobs currently pending in an account:

sup fax_maintenance account_jobs {ACCOUNT_ID}