Monitor bandwidth usage per application or IP

There are a few simple text-based tools that you can use:

• bwm-ng shows numerical counters for traffic rate on every interface (rx, tx and total) at a configurable refresh rate (default of 500ms)
• iftop shows numerical and bar graph indicators of traffic based on source/destination hosts
• iptraf shows numeric packet and byte counts based on connections (source/dest host and port) or packet counts per interface, port or physical address.

(all should be available for common distros, but maybe not in the default repos)

In my personal experience, if call quality is the problem, you should look more into latency and jitter than bandwidth. Asterisk (if that's what you're using) has some nice builtin tools and debugging, and some other network tools can help.

If you want to know bandwidth, it would probably be better to use something like Cacti or Munin or something else based on RRDtool to graph the statistics over time.

I looked up the specs for the SonicWall TZ200 and it says that it can create Netflow accounting packets. Consequently, any netflow collector. Personally, I've used nTop before and I know it does IP based bandwidth usage , but there are tons of alternatives out there if that doesn't fit your requirements.