You don't state whether you call using a "land-line" or a cell phone. A greeting is a recording and it may be different than what an actual connection would be when speaking to a person.
Having said the above, I have never had an issue with the call quality in either contacting RingCentral or with my RingCentral account. RingCentral call quality, as with any VOIP service, can be enhanced by correctly configuring your network. Your local network, your Internet connection, and your router devices all contribute to overall call quality.