Mmm.. good question, and one that can be answered by setting it up and logging into the router (telnet, ssh) to verify the Linux routing/QoS setup afterward (most Linksys and other routers are Linux-based).

But even that should be unnecessary. QoS on Linux gets applied to the upstream link (or links on a bridge), not the downstream. And the DMZ is downstream. So if the VoIP ATA is plugged directly to to the Linksys unit, it should work as well as it ever will.

But that's another story. QoS is difficult/impossible to do perfectly when one has control over only one end of the link. Unless your ISP cooperates (rare/unlikely), then no product at your end can do a 100% perfect job.

The problem is the ISP downstream connection, which queues up packets on the ISP side. At your end, you cannot control that, and therefore cannot do full QoS on downstream packets. Upstream, yes, piece-o-cake.

What Linux solutions tend to do, is flow-control the downstream, trying to target a bandwidth that is 10-15% below peak connection capability. The reserve bandwidth leaves room for high-priority (VoIP) packets.

All of the Linksys / whatever equipment all do it that way.

EDIT: A possible exception is when the modem itself implements QoS (eg. SpeedTouch 780WL/voip). There's a way to do QoS at the ATM cell level, which only modems have access to. But whether it can do it or not (for downstream traffic) depends on the DSLAM (telco/cable) side of the connection.

Cheers