What's the secret to operating voip across cable/dsl links successfully?

How do call centers that have all work-at-home employees operate successfully? Usually they have residential grade Cable/Dsl connections, and a VPN router that tunnels back to the mother ship.

I can't comprehend how VoIP can function in an environment like that, where packets traversing these consumer grade broadband networks are highly, highly prone to out-of-order packets, high latency, high loss, high jitter.. you name it!

We also know that VoIP is one of those technologies where if you do have these problems, the users will always complain, and if your job is to take phone calls 24/7, not being able to hear the other person, or have them hear you--is a very big deal!

Yet a lot and a lot of these companies are doing this. What is their secret?

There's no QoS across the Internet...