RingCentral API Fax in developer account is used for demo purpose only.

Sandbox environment is used for testing purposes only . It will have an image watermark of "Test fax using a RingCentral Developer account', :

https://developers.ringcentral.com/guide/basics/sandbox

https://forums.developers.ringcentral.com/questions/941/fax-coverpagetext-removed-after-some-special-chara.html

So even if you send to a real number that will have this test messages.

But even if you don't want to use a real number and get a mock response, I am not sure RingCentral Fax API supports that.

You can create your own API url and put mock response to do something like that.

You can even contact backend developer support team to get an idea of mock or stimulated response with actual RingCentral API without sending fax

You question is answered in the below link.

https://stackoverflow.com/questions/49864445/how-to-test-ringcentral-fax-api-without-sending-to-real-fax-machine

ok, so basically from the link @Khadhar Koneti has shared here:

https://stackoverflow.com/questions/49864445/how-to-test-ringcentral-fax-api-without-sending-to-real-fax-machine

seems that the solution mentioned there is to create two sandbox fax numbers and send from one number to the other. Since, sandbox account provides multiple numbers, this seems too be a good way to test the fax and also receive the fax without using real number or machine device.