I agree with Eira.
It may not seem important to you, but you might not have seen the fast moving millitary traffic converging at the same level. The increasingly frantic atco has, and also has their license flashing before their eyes - he/she has very particular legal responsibilities once you are in receipt of a service, even if it's 'only' a FIS.
Also, if you are unable to respond to a request to confirm your level, or negotiate a course of action ( 9 times out of 10 what you were intending to do anyway, but we need to check ) then you might be causing major hassles to the atco who is trying to work IFR and/or heavy jet traffic around you.
I have to qualify the above - the odd missed call doesn't matter in the least. Atco's (mostly) appreciate that it is a lot noisier in your cockpit than it is in the tower. They also realise that you can be busy or distracted. The problem is the pilots who habitually call up for a service and then turn down the volume because they can't be bothered listening, and then bumble through an instrument approach leaving havoc in their wake. At the end of the day, why request a service if you don't want to keep your end of the bargain?