RingCentral would be able to sell more fax numbers by implementing this, because businesses would want a pool of phone numbers to send FROM. In our business (and I'm sure others), faxes often fail to send to the recipient (I think it's probably a bandwidth problem on the recipient side).
Another possibility is that the businesses we're faxing (Credit Bureaus that want to work against us and our consumer clients) are blocking our phone numbers.
Two Potential Implementations:
1. Buy and select a pool of fax number Caller IDs to round robin when sending faxes. This would be set in admin --> user management --> outbound calls/faxes --> fax settings.
2. When sending a fax via Zapier, email, or API, allow the client to specify what fax number/caller id to send from.
Additional Related Features:
1. Allow a setting to retry a failed fax N times. With each attempt it sends from the next fax number/caller id in the round robin.