Audio problems on cable modem link

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We have a small office installation running over a cable modem. (8M down, 500k up confirmed with numerous speed test sites)

When a single call is up, call quality is fine. When a second call is up, outbound audio is immediately choppy. We’re using ulaw, and confirmed that traffic with 2 calls is <175kbps in/out. (IAX connection out) Asterisk doesn't report any dropped frames, the internet connection looks fine, etc. We have a linux router in place running wondershaper that seems to be running fine (same as our other installations). Can someone suggest where to look? Could this be the ITSP?