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You're correct that there are many variables to figure out what is affecting your VoIP service. Essentially, if your network doesn't have enough bandwidth to support the data stream for the calls you're making, quality is going to suffer.
Depending on your provider, you should have some console that allows you adjust various components of your service. However, you may want to check your network first as it is the foundation for the communication service.
Start by using this:
The quantifiable, general measurement of a VoIP service is you MOS or mean opinion score. It ranges from a 1 being awful to 5 being very good. In a nutshell, this is measurment is based on several network factors like upload and download speeds, jitter, latency and packet loss.
Check with your provider for more details on how you can tweak your system to improve call quality after you are certain the network is in good condition.