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E.164 format is an internationally accepted format that allows international communication to avoid any sort of incorrect interpretation. So as RingCentral uses in their calls and sms, fax etc.

ref: https://medium.com/@samyukta.g.bharadwaj/importance-of-e-164-format-5f059be1d61e

"The API supports phone numbers in e.164 format with plus sign + complying with the ITU-T recommendation. "

ref: https://developers.ringcentral.com/api-reference/Phone-Numbers

Also check this: https://support.ringcentral.com/s/article/Outbound-Calls-Destination-Number-Format?language=en_US

It should be in E.164 format. But for number in the U.S, it works w/o the country code too.