Why should they be expected to have the manpower to answer every single one?
Because they are expected to consider the opinions of their constituents, and as an elected representative and they are supposed to representour interests, not theirs.
To be able to functionally listen to your constituents, yes, you need the staff to be able to do it.
You think that listening to and transcribing voicemails somehow takes less time than taking a call? Pro-tip, they take about the same amount of time.
I mean, with that attitude, why have any staff at all? Just let it all go to voicemail and never read any of those thousands of emails from constituents! Fuck 'em! Fuck representative democracy, amirite?