I have very freaky cable internet


I'm paying for 1gig/50mbps but I've never seen more than 30mbps up.
In the evening the upload speed drops to about 20mbps and the download speed drops to about 150mbps, so setting up traffic shaping rules is tricky.
My main issue is with voip. We have 6 voip channels and 6 extensions connected to Anveo on port 5010 (sip.ca.anveo.com) using g711 (about 87kbps per channel so says the traffic queue). When doing big downloads using sftp (48 threads) it murders the voip (timeouts, dropped packets, big latency, etc.).
What I would like to do is set up some type of rule that once it detects a voip call it throttles everything else to 1% or something. When I'm on a call I don't need to be using the internet. We don't have IPTV or anything else that needs priority.
-Jamie M.