Ratio of calls to operators. If people are unwell, and/or the place is understaffed, you have a high volume of calls. Low employee retention plays into this too, as it is the cause of being understaffed (and budget), because who wants to stay in a call centre? Especially in gov where you can move around with relative ease.

But also, that's just the gub'mint for ya

I heard they feed multiple calls into the same line with the hold music. It means they can have more people on hold with less lines, but the quality of the line drops with each call added to it.

But that was in the old days, you'd think everything would be internet based now

It's better business to waste your time rather than theirs. Twenty years ago they'd offer to send people home without pay on quiet nights at my call centre. Often it'd get mad busy half an hour later.