How to Fax over VoIP on the Internet

Brightstar wrote: “I keep hearing about how hard it is for faxes to work through VoIP (Voice over Internet Protocol). I’ve prepared a top five list of things you can do to make sure your faxes have the best chance of going through.”

  • Set your baud rate to 9600 and turn off Error Correction Mode. The slower your fax goes, the more likely it is to go through without errors. Most VoIP providers are unable to support the default baud of 33600 baud due to the amount of data that has to go through. Error Correction Mode was designed for faxing PSTN to PSTN. Voice over IP often has extra noise added due to jitter instabilities in the connection as it travels from one fax to another. This causes false positives for Error Correction Mode because it will cause the fax to loop over and over trying to fix these noise errors, when it can’t be fixed. This will end in a communication error usually.
  • Use a provider that supports T.38 – T.38 is an audio codec designed for faxing over VoIP. it allows for more detail and better looking images as well as stability by creating redundancy in the RTP stream.
  • Set your packetization to 10 milliseconds or less. The more header there is in the stream, the less likely jitter and instabilities in your internet connection are going to cause a synchronization error. (You may need to contact your provider to make this happen if your box is locked, such as if you use Vonage.)
  • Keep on top of your ISP. Stability in your Internet connection are paramount in making sure that faxes are successful. Speed tests can help you track this. Look for an advanced speed test that offers more than just and upload and download speed. A personal favorite of mine is located at MyVoipSpeed (Note: An up to date Java version is required) which is really nice because once you’re done, you can click on the detailed analysis link and then call up your ISP and give them the URL for the test so they can see what’s going on.
  • When all else fails, call your VoIP provider. There may be issues in the routing of your calls that are causing problems. Only your VoIP provider’s call processing team will be able to fix this kind of thing. Have your provider test from a PSTN line and then their own VoIP line. If they can send through PSTN but not their own VoIP line, it’s a routing issue on their own end.
  • Bonus Tip! Don’t even think about faxing using Voice over IP on a satellite or wireless connection. These kinds of connections are too unpredictable, and add a huge amount of latency that will make it impossible for fax machines to sync up properly.