Just like to say that in my experience; (Europe, USA, M East, India, Africa, Caribbean), UK ATC have the clearest transmissions (and best microphones). Before becoming a pilot I was a BBC engineer, and I despair at some other countries' ATC transmission quality !
Many have distorted transmissions and the trained ear can tell that their microphones are full of crud and spit. The transmissions are distorted and the noise cancellation has ceased to work, so you either barely make out what ATC is saying, or it sounds as if they are transmitting from the toilet or the works canteen !
North Western Europe is mostly all good, but the further East and South one goes, the worse it becomes.
What tests, if any, are performed to check microphone quality and performance in the Uk ?