A navigator friend got some NAAFI coffee granules and mixed them with cow gum (charts for the joining of). He smeared the result on the earpiece of his boss's phone. He stood in the ops room until he saw his boss arrive and go upstairs to his office. He gave him a couple of minutes and then rang his number.
"Good morning Sir, GPO here, we're getting reports of clients getting brown sticky ears from our telephones"!
Went down well.