The voice calls usually share the internet connection with general business traffic.  In most cases this doesn't present a problem.  However if there is very high load from non-voice traffic, this can use up all of the available internet bandwidth impacting voice quality.  To generate sustained traffic that can saturate connections 30mb or faster requires exceptionally high use.  Watching a netflix HD movie uses around 5mb download, Youtube videos less.  Regular browsing and email uses some bandwidth. Companies that move around large design or video files may use high bandwidth from time to time, but to consistently use a high bandwidth is unusual.  This may be a backup of huge files (eg syncing to icloud or google drive or large amounts of data), or this can sometimes can be malware running on a PC that is being used for some nefarious purpose.  Sometimes audio issues are caused by a device on your network consisting  using all of your available bandwidth. 

Some more advanced routers have graphing and logs that can hep identify internet use by device, and network experst can use analysis tools to watch the traffic, but there are few simple tests that most users can perform.  Often, these will identify the problem.

Step 1 - do a Speed Test.  This will tell you the maximum upload and download speed

Step 2 - do a Ping Test .  This test tests the quality of the connection.  If the ping test is marginal, a lot of variability or packet loss there may be a device on your network that is using most of your internet bandwidth

Identifying which device may be causing the problems requires trial and error.  Leave the ping window running and this takes a process of elimination.

Wireless Devices? 

With the ping test running, login to your router and disable the wifi.  Make sure that the computer that is running the ping test is using a wired connection, otherwise the ping test will stop.  If the ping becomes more consistent when the wifi is disabled, chances are that one of the devices connected by wifi is causing the problem.  To locate exactly which device, re-enable the wifi on your router and then disable the wifi on each device.  Again, you are looking for the ping to become more consistent when you disable wireless on a device. 

Which Wired Device?

If you have established that it is not a wireless devcie on your network, finding if it is a wired device will also need some trial and error.  Most network switches have a light that flashes for each port when there is data traffic.  Sometimes, the rogue device consistently using high bandwidth can be identified as the port that has a constantly flickering light.  Starting with the ports on your switch that are flashing fastest,unplug the network cables into your switch one port at a time while watching the ping test you have running. You are looking for the ping to become more consistent when you disconnect the network cable for a device.

What to Do if You Find One  Devcie is Causing the Problem

Once you have identified which device is causing the problem, check it out to see if a large backup or sync job is running to a cloud storage servcie, or run a virus and malware scanner.