You can see how good businesses start out positive and cheery toward their customers and eventually turn full circle. Grinds you down after a while.
Keep up the outstanding work mate - some customers can't be helped and nowadays know they have an instant online outlet to vent. Almost legal defamation in a way!
Absolutely, but the funny thing is that you treat each caller equally, give them your time and attention. It is part of the job. On the occasion, an idiot (as described on the OP) will assume that you have to drop everything, or they will go to social media to vent and carry on with deliberate intent to cause trouble.
One of the biggest issues I find doing this type of work
is dealing with Humans
Don't forget that some customers may be hard to deal with as they have been screwed over before. There are some unscrupulous service providers out there.
But most hard people are just tossers 
At least 2 calls per day. Newbies burnt by overpriced and poorly "shopped" machines. Parts which are advertised as new, which are shop soiled old parts. Poorly made ramps, protectors even fake side art. Each day brings something new, which nowadays, never surprises me. My favorite is a "fully done up" machine that "hummed" when it was switched on. The "professional" dealer had configured the game with a Japanese power configuration (218 VAC). Also, had a NEW 10 AMP SloBlo "mains" fuse. The poorly "shopped" Addams with the original pop bumper skirts and bodies for $9,000 was atrocious. That's NINE THOUSAND - complete with the front door being painted while still in the machine !
I hear some stories, and I'm not at all surprised that customers that call have been angry for quite some time..
