Testing fax without real fax machine

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.