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1. Is that true even if sending more than one fax to the same external fax number (at the same time) from different FROM fax numbers ?

Yes, each time you call the "fax" API, it is an individual fax message. It does not matter if it is from the same or different number. The only thing for you to take care is that in order to sen from different numbers, you have to login your app with different users.

@Phong Vu You mentioned it would be busy for a second fax if there were two inbound faxes at the same time. Is this still the case? This page https://www.ringcentral.com/fax/features/how-it-works.html#accordeon-4 under "never get busy signals" states "Receive faxes simultaneously and get instant alerts by SMS, email, on your computer —even if you’re on the phone. " how is it possible to utilize this capability so we don't have to worry about more than 1 fax being transmitted to us at the same time?

I am not sure about that statement. I don't have the specs so I cannot confirm how it should works for inbound faxes. It also takes lots of efforts to test to verify this. I can't remember my tests cases. But if you still observed that the line is busy then it is the same situation.