Low, everyone has annoying customers. I have one lady who will call and call. I tell them the same as you, when the part comes in i will call. I tell them everyone gets parts from same place, and can't get them faster. Not much you can do.
As for you Patricio, I just tell them that the problem they have is a different one from the one they have now. On my invoice, I have "30 days labor, 90 days on parts" I'll pull the invoice if they get into a pissing match with me. I have one apartment owner, always calls and says I just fixed something and now it's broke again. I always tell them - If it is something I fixed and its still under warranty period, It'll be taken care of. But most of the time, it's something else. Don't let them bully you.