Receiving faxes from VoIP Providers

Incoming faxes from VoIP Providers can be quite tricky when compared to receiving of faxes from PSTN, ISDN VoIP Gateways. The main reason for this is NAT. Therefore some tweaks need to be made to the 3CX Fax Service Configuration Files.

• Some VoIP Providers do not need this setting but others will not work without it.  The best thing to do is to leave the configuration unchanged and try with the default configuration files as shipped by 3CX. If you receive a Fax with no pages, then you will need to modify the configuration files as described below.
• If you receive a 488 NOT ACCEPTABLE HERE message code, modifying the configuration files will not help.  You will need to contact your provider to confirm whether T38 is supported.

Pre-requisites

Alternatively in Version 10 and up of 3CX Phone System you can create FAX MACHINES. These are special extensions designed especially for FAX receiving. They already have the FAXDIRECTSDP option enabled internally.

Also these extensions never deliver audio via the 3CX Media Server making it possible for FAX Transmission to work correctly.

Part2: Configuring the 3CX Fax Server global templates

This section will explain the files that need modification. These are the following: faxglb.cfg and in_session.cfg.

1. From 3CX Management Console, go to “Settings / Provisioning Templates”
2. Select Template Type “3CX Fax Server”.
3. Select Template Name “faxglb.cfg” and click edit
4. In the [SIP] section do the following:
   • comment the “NAT support = FALSE” option by adding a “;” at the beginning of the line
   • remove the “;” in front of “NAT support = TRUE” to enable NAT support
   • remove the “;” in front of “(NAT) local SIP IP port = 5100″ to enable this setting
5. The extract below is an indication of how the “faxglb.cfg”should look like
   

   

6. Click on the “Apply” button. A dialog box will appear asking you to save this file. Keep the file name the same and click “OK”.
   You will be asked to override this file. Click Yes. At this point faxglb.cfg is complete.
7. Go to “Settings / Provisioning Templates” and this time select the template Type “3CX Fax Server” / Template name “in_session.cfg” and click edit.
8. Many providers will reject fax calls with “488 Media Unavailable” if both UDP and TCP are specified as options for T.38 Fax communications. To help improve interoperability we can restrict T.38 to UDP only.
   To fix this do the following in the [General] section:
   • comment out the option “kind of T.38 transport = UDP_TCP” line by inserting a “;” in front and
   • remove the “;” in front of “kind of T.38 transport = UDP” to enable this setting.
9. We now need to configure NAT traversal functionality for the Fax Server. In the [T.38 media address] section do the following:
   • remove the “;” in front of the “local media ip address” line and replace x.x.x.x with the local IP Address (In this case 10.172.0.15)
   • remove the “;” in front of the “local media ip port = 10000″ line
   • remove the “;” in front of the “(nat) local media ip address” line and replace y.y.y.y with the Public IP address (In this case 78.158.130.232)
   • remove the “;” in front of the “(nat) local media ip port = 10000″ line
10. Click on the “Apply” button. A dialog box will appear asking you to save this file. Keep the file name the same and click “OK”.
    You will be asked to override this file. Click YES.