No sound on some calls using a fiber phone line

From time to time when we answer an incoming call (which is routed through RingCentral to our fiber landline phone #) we cannot hear the caller. For a lengthy period of time, we just chalked it up to the caller probably using a mobile connection that was terrible. But it is happening a handful of times a day now, and it doesn't seem reasonable that this is the reason so often. We're puzzled since our landline is fiberoptic and we don't have this problem with outbound calls (which aren't routed through RingCentral). Ideas? Thanks!