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

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