Test your bandwidth. 

Typically, your true upload and download bandwidths should be at least 80% of your what your service provider's level of service states it is.  Testing your bandwidth is easy and should be done at least occasionally, but if your VoIP quality drops then testing your bandwidth speeds should be one of your first tests. 

There are many sites available for testing bandwidth uploads and downloads.  One of which would be DSL Reports. 

Knowing the bandwidths (upload & download) that you should be getting is important so you will be able to know how the actual measurements relate to those levels.  They should be at least 80% using a valid test.  A valid test should be one conducted without applications (including VoIP) running while the bandwidth test is in progress.

If your bandwidth speeds fall short, then you should work with your ISP to correct the issues, which will improve both your bandwidth and make for a better connection and better VoIP quality.   There are conditions with modem signal levels which will affect your overall connection and should be corrected if discovered.  See more about cable modem levels here.  In some cases signal levels that are at unacceptable levels can cause the modem to struggle to keep connected and result in brief periods where connections drop.  This type of condition would lead to conversations completely disappearing or dropping.  (Having enough bandwidth (level of service) is critical for VoIP and should have been one of your first criteria's when having started service.)

Test your VoIP connection.

Having tested your connection for packet loss, extreme jitter and bursts of packet loss from a ping test, and checked your bandwidth, a next test would be to do a more complete MOS style test.  These tests measure a number of values that will affect VoIP and give a value which can be related to the expected voice quality of your connection. 

Run a test on your VoIP quality.

Pick the codec that your connection uses.  (If you are not aware of which codec you might be using check with your VoIP provider or pick G711.)  After you complete the test, picking a server that is geographically close, you will be given some good information. 

 

A summery includes 3 important items:

Jitter

Packet loss

MOS score

 

 

One factor that a VoIP test will measure is jitter, in which small variations will impact call quality.  ATAs and IP phones are capable of handling very small variations in jitter by the use of buffering, but once the level exceeds what they can cope with begin quickly to degrade voice quality.  For a description of voice quality and how it relates to a MOS score read this.