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There are many ways to send a test fax. Each carrier has a special test fax number. Some large printer manufacturers also have fax test numbers,such as HP and so on.You can read more from this https://gmailtofax.com/

If you check this:

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.