Responding to potsman: I have tried multiple phones. The same phones that were doing fine over a 1500 foot or so copper pair to the local telco. For my install, there's no reason for loop current to be boosted, though I agree that there isn't necessarily a causal relationship between elevated loop current and too-high earpiece volume. Here's an overall discussion of the topic:
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www.sandman.com/loopcur.htmlUn-Boosting loop current alas seems unlikely to reduce earpiece volume. Some E-MTAs allow control over receive and transmit audio gain. If RCN were able and willing to adjust receive audio gain on the TM822G, that might help my situation. Otherwise, an in-line attenuator appears to be in my future.
As far as Pacomartin's insight regarding wideband audio, I wonder if that still comes into play when the other end of a call is POTS.