Convince them to go all VoIP. PSTN will go away as soon as incumbents can get out from under federal requirements. If you are concerned about call quality, talk to your ISP and see what is available.
VoIP has come a long ways even in the past year; call quality on a decent link is phenomenal- much, much better than PSTN. I understand there are technical requirements for PSTN in some situations, but my personal take is go VoIP end to end- far easier to manage and better quality.
I agree with you, thing is that I'd have to use their DSL connection for the VoIP and then setup QoS on the router for voice. And, maybe it's just in my head, but PSTN seems to be a lot more reliable than VoIP, at least here in Canada.
Is that the normal/best practice at small businesses with VoIP? One internet line coming in that's shared between data and VoIP?
edit: ok I'm looking at my ISP's small biz stuff now and they have something called IP trunking... how is this different than another data line connection?