VOIP locks blocks cable modem uploads

Hello,

Customer has Windows XP Pro.Dell Dimension 4400. There is cable modem. The cable modem connects to a motorolla vt 1005v VOIP device. The computer connects to the voip device. This device takes the signal from an analogue phone and transfers it to digital and sends it out through the cable modem.

When the phone is in use, uploads over the cable modem from the XP machine hang–as soon as the phone is hung up, the upload completes. We can download while using the phone.

The motorola device and cable modem have been replaced with known good devices. There is no software installed on the PC for this device or the cable modem.

On the xp machine we’ve done the following:

Tested with safe mode with networknig. Same problem.
Disabled QOS, same problem.
Updated drivers for NIC, same problem.
Reinstalled windows (repair install). Same problem.

Thanks for any advice,

Josh