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In general Fax can be viewed in the Service Web Portal (https://service.ringcentral.com) in both Messages > Sent Items and in the Call Log.

If a fax transmission fails, the reason is reported in the Call Log Record's result property. It is also presented in the Service Web Portal. All the Fax question are answered here and the following link will be helpful:

ref: https://forums.developers.ringcentral.com/questions/144/fax-faq.html

ref: https://forums.developers.ringcentral.com/questions/1350/i-need-help-diagnosing-an-undelivered-fax.html

https://forums.developers.ringcentral.com/questions/150/how-to-know-if-a-fax-transmission-through-the-api.html