5) Line noise.
Having naff connections inside your house will not help
Fitting the faceplate (as per 1 above) will help a lot, as it cuts the 'phone' side away from the b/band side.
But if when you pick up your phone you gets crackles and hisses and pops, it's not a good sign
Unplug ALL your phones, then take one, and plug it into each extension in turn - when you find the culprit, it's time to do something about it! (but that's a bit detailed for here)