1) Welcome to the party!
2) when you post, please post your entire configuration including (but not limited to) your installation method and vicidial version with build.
this IS a requirement for posting along with reading the stickies (at the top of each forum) and the manager's manual (available on EFLO.net, both free and paid versions)
You should also post: Asterisk version, telephony hardware (model number is helpful here), cluster information if you have one, and whether any other software is installed in the box. If your installation method is "from scratch" you must post your operating system and should also post the .iso version from which you installed your original operating system. If your installation is "Hosted" list the site name of the host.
If this is a "Cloud" or "Virtual" server, please note the technology involved along with the version of that techology (ie: VMware Server Version 2.0.2). If it is not, merely stating the Motherboard model # and CPU would be helpful.
Similar to This:
Vicibox X.X from .iso | Vicidial X.X.X-XXX Build XXXXXX-XXXX | Asterisk X.X.X | Single Server | No Digium/Sangoma Hardware | No Extra Software After Installation | Intel DG35EC | Core2Quad Q6600
3) Any router will do, normally, but you'll need to turn off all internet surfing for your agents. If possible, you should get your Vicidial server its own IP address that does not go through a router. This will remove the need for QoS entirely (but require the firewall in the Vicidial server to be working properly! Full lockdown!). Then you can turn off internet for your agents by unplugging the router and see if that improves call quality. If it does, your problem is agents on the internet and/or infected computers.
4) I also note that you say "15mb down through" ... which is only 50% of the information regarding the internet connection, and it's not the "necessary" part! You need to have fast Upload. Use
http://speakeasy.net/speedtest/ or
http://www.broadband.gov/qualitytest/about/ to test your speed in both directions. And test it a lot, because if it varies you could have equipment problems.
5) 10-20 person call center ... isn't very informative. How many simultaneous calls do you have going through your ISP at the most? That's the bandwidth requirement. Multiply each call by 100k and you'll find your number. If you have less than that, you have a problem. LOL (g729 allows for about 30k instead of 100k, but it's not free!)