I had to walk off crying

I work for a mobile network as a sales advisor. I set up phone contracts and sell handsets. Occasionally customers come in with broken phones so I do what I can to fix them (even though this is not my job, there is a repair helpline.)
So this one customer comes in with his wife claiming both their phones are broken. I have seen him before and already advised to ring the repair helpine as the phone is broken beyond my skill to repair it. Apparently this is “ridiculous” and because I sold him the phone I should be able to fix it..
Even though I am not required to do anything for the customer in this situation, I am a nice person so I ring the repair team for them. This goes on for about 10 minutes as the process is explained. The customers are unhappy that a charge may apply (because they’ve obviously broken the phone themselves) and they start to scream and shout at me. My manager isn’t in that day so my colleague finds another manager. The customers continue to shout at me until I just crumble and cry. Luckily the manager appears at this point and I just have to walk off into the stock room. I’ve never been so angry and humiliated in my life… What is wrong with people?