Fax Voip is a Windows based software that provides a full-featured SIP/H.323 softphone for Voice over IP (VoIP) calling, and also provides full functionality for sending & receiving facsimile transmissions via T.38 & G.711. From the perspective of your fax or voice application, it acts as a Voice/Fax modem, installing Virtual COM Ports and Virtual Fax Voip 14.4k TAPI Voice-Fax modems on your system. You can configure your Fax & Voice program to operate with both the Fax Voip COM ports or virtual modems. From the perspective of your VoIP network, it provides a fully featured SIP/H.323 client with support for faxing over VoIP trunks, both in G.711 and T.38 (and more importantly, with Error Correction Mode - ECM!)

T38fax.com has officially certified this software after completing rigorous testing, designed to push both the software and the virtual interfaces to their limits. We're pleased to report that the Fax Voip software has passed our requirements to make it to our Certified Device List. We’ve included a simple configuration guide on this page to help you get up and running. However, if you're running into trouble configuring your Fax Voip T38 Fax & Voice software, please open a ticket and one of our Engineers will assist you with your configuration as soon as possible!

If you don't have an account with us, that's ok! We offer a 30-day free trial that includes a free $25 credit to your account and one free DID to allow you to receive faxes. Click here if you'd like to signup for our Free Trial and take advantage of these awesome freebies!

Configuring Fax Voip with Your T38Fax.com SIP Trunk

This guide provides instructions to configure Fax Voip Software's T38 Fax & Voice software to utilize our T.38 optimized SIP Trunk service. More information on Fax Voip Software and their products can be found on their Website. If after reading this guide you still feel that you need assistance, please open a support ticket and one of our Engineers will be happy to assist you as soon as possible!

A common question we get is whether Fax Voip T38 Fax & Voice can be used with Microsoft Fax & Scan. Yes, it can, since it installs virtual modems which appear just like normal modems to Fax & Scan. This is currently the preferred method to use Microsoft Fax & Scan with our services, since Fax Voip FSP is not yet certified for our platform.

  • Check your version. These instructions are for v.7.6.1+.
  • Run the Install executable as an administrator.
  • Make sure to fill out your modem information, or your modems won’t install correctly.
  • Open the “Fax Voip Control Panel” program.

Configure your SIP Account

Configure Outgoing Call Routing

  • In the sidebar, go to VOIP > Outgoing Plan.
    • Select  “New”
      • Ensure you’re in the “General” Tab.
        • In the “Call from Fax Voip Line” section, select the following radio dial options:
          • “From built-in lines (via Fax Voip Console)”
          • “If to send a fax with T.38 support (VOIP)”
        • In the “Dial on Line” section:
    • Select “OK.”

Configure Incoming Call Routing

  • In the sidebar, go to VOIP > Incoming Plan
    • Edit the “VOIP Call From” “sip:.*” rule
      • In the “Call To Fax Voip Line” section, select the following radio dial options:
        • “Receive Faxes as Files”
        • “Fax Protocol: T.38 Support”
    • Select “OK.”

Configure Fax Settings

  • In the sidebar, go to Fax.
    • Set “Default Fax Mode” to “T.38 (auto).”
  • In the sidebar, go to Fax > ECM/Rate.
    • In the “Built-In Lines” section:
      • In the “T.38 fax” row:
        • Check “ECM” in the “Reception” column.
        • Check “ECM” in the “Transmission” column.
  • In the sidebar, go to VOIP > SIP > T.38.
    • Set “Indication” to 5.
    • Set “LS-redundancy” to 3.
    • Set “HS-redundancy” to 3.
  • Click “Apply” at the bottom, in the “Fax Voip Service” section.
  • Click “START” at the bottom, in the “Fax Voip Service” section. (or “STOP”, and then “START”).

(You will also want to set up your inbound routing here, under Fax > Incoming Methods (Built In…)). 


  • Open the “Fax Voip Console” application.
  • In the top left corner, select “New Fax.”
  • Select a test fax page. When in doubt, check for “Fax\Inbox\WelcomeFax.tif” in your home directory.
  • In the Fax Number field, type in 12158258792. This is our faxback number.
  • Click the “Add” button.
  • Under “Fax Protocol,” set the radio dial to “T.38 support.”
  • Hit “Send Fax …”