QoS won't help.
Static/noise is an analog problem, not a digital one. If it was a network problem, you'd be experiencing audio dropouts or studdering or glitches but not "noise".
There are only two places where the audio is analog, in the set and at the pstn gateway.
And to be clear, when you say "noise", you can still hear the other person but it's a staticy call, right? Not a call that is completely static (that's usually the result of mismatched encryption and decryption keys).