VOIP = Voice over IP, digital lines, internet phones -- Ring Central is VOIP, cellphones are VOIP
POTS = Plain Old Telephone Service -- traditional landlines; uses copper pairs
As the Ring Central documentation carefully explains, Faxing over VOIP has issues and customers with intensive fax requirements should stick with a POTS line.
Our company's reliance on faxing has diminished considerably so we are taking the plunge and going all-VOIP.
Here is an expectation table I derived from my testing of our small (<10) user Ring Central system that some may find useful:
To: RC fax # (VOIP) POTS fax number
POTS fax machine works fine traditional faxing not relevant to RC
RC # autodetects
fax call and processes
VOIP fax machine may not work works fine especially if on same RC account