People sometimes ask me if I tested the cable and I just ask them, does your internet and email work? If it does than don't need to test the cable.
I have ran voip on cat 3 believe it or not, I don't recomend it, that was just a test to see if it would work and was a very short run. We put it on a cat 6 in the end.
We have alot of cat 5 and 5e and have never had issues with cable problems and some of this cable is over 10 years old now.
Again, most degradation problems will be in your routers, switches and make sure you have fiber in place where needed.
A good backbone is the foundation of a successful voip operation.