It's now an older technology, but for super voice quality, no drop-
outs, no crackles, no delays - you need ISDN.
I had it for a while when broadband was unavailable here (home
highway), data rate nothing special (but reliable and near-instant
connection) - but voice calls connect (as afar as you can tell)
instantly, with an "in the same room" quality if it's end to end ISDN,
and pretty good if it's ISDN to analogue. You also get 3 different
numbers you can use, which may be useful.
Not a cheap solution, but if this is the basis of your business, then
the additional cost might be trivial to you. However you would need a
dedicated line - you can't have IDSN and broadband on the same line.