1) Welcome to the Party!
2) As you are obviously new here, I have some suggestions to help us all help you:
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) Moved to "support" as this is obviously a support question.
4) The description of your problem is a bit vague. Are you saying 1 out of 5000 calls has unusual sound while you are talking with your customer, at a random moment? Or are you saying that every other call will actually be made to two places (one a business and one a residence)? (I Know, neither fits your description, but I'd like to push you to be more detailed ... do a little tracking and see if you can find a rhyme or pattern ...).
5) Overloaded systems have been known to drop extra calls in agent sessions unexpectedly. Post the results from "uptime" on the command line which shows simple 1,5,10 minute Average Server Load. You can also use an app called htop to show that same information plus a bunch more. Best part about that is it will show you a bar chart with all processors on it so you can see if individual processors are maxing out often. Note that the number of cores in your system should be double the max average server load. IE: If you have 4 cores, average server load should not exceed 2.0.
6) Have you tried another carrier? If it is a voip problem ... that simple test will tell you immediately. Always have at least two (and we recommend three!) carriers available and configured. Changing carriers is a common first (simple) troubleshooting step!
Happy Hunting!