Sending fax using SIP

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If you have a subscription with a SIP provider that supports T38, you can use Kapanga softphone to send fax from your computer without a fax machine. For testing purpose, I would suggest flowroute which gives US$0.25 trial credits upon free signup.

To send a test fax, download and install Kapanga softphone. It is assumed that flowroute is used as the SIP provider

1. Right click on the softphone UI, choose Call Control Settings

2. Open SIP paramaters tab and input your account username. You can put anything for the display name.

3. Open Proxy Configuration tab and configure as follows:

SIP proxy:
Authorization User: enter your flowroute username (e.g. 5xxxxxxx) and password
Tick the following checkboxes: Register, Register Automatically, Proxy Keep Alive

4. Right click on the softphone UI, choose Fax Control Settings. This step is important; the default Fax settings may not work with flowroute.

In General Settings, tick the following checkboxes: Re-send last packet, Error Correction Mode, Re-use RTP port, Send CSI/TSI.
Set Preferred Resolution to MR and check Negotiate Resolution
Set Preferred Data Rate to 9600bps, either V.29 or V.17
In Send/Receiving Faxes>Outgoing Faxes settings, tick the following checkboxes: Prompt to send faxes, Prompt before call

5. You are now ready to send your first fax. Enter the number to fax to. You will be prompted to select a PDF file to fax to. Upon connected, press the Fax button to start fax transmission. If everything goes fine, you should see the softphone transmitting the fax as in the following screenshot:

A 1-page fax will take around 3 minutes to complete at 9600 bps. After you have sent your first fax successfully, you may want to tweak the settings for higher fax resolution and transmission speed.

Kapanga softphone supports incoming fax as well. To try out, register 2 flowroute account and sign on to both accounts from different kapanga clients. Then, from one client send a test fax to the other client’s flowroute account number. To receive incoming PSTN fax, you need to have a DID number that forwards all incoming call to your Kapanga softphone, refer to this post for more information.

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A tough developer who likes to work on just about anything, from software development to electronics, and share his knowledge with the rest of the world.

One thought on “Sending fax using SIP

  • November 21, 2017 at 10:17 am

    I’m trying to get this to work with but I can’t it it to register. The inbound and outbound proxy is I have as follows:


    And then I make sure to put in my username and password. It still won’t register. What do I do? I can register it fine on my ATA. I even made sure I unregistered it on my ATA first so there are no conflicts.

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