What are warranties actually for?

On the 29th my phone was left in the car, upon retrieving it, it was not coming on. We took it into Voda4u at liberty mall as that is where we purchased the phone. They then sent the phone to their repair shop. Who phoned me and just gave me a quote for the repair. When I told them that the phone is only 3 months old they then said I need to produce a proof of purchase. Which I then asked them why I should. Could they not get that from the shop that sent them the phone. To which they rep**** that they don’t have my details so no I need to do it. (but they phoned me) and they also have a job card with all my details on it. Anyway I then got hold of wandile at the Voda4u who sent them my proof of purchase. When they received this all of a sudden the repair was not under warranty. Could they not have said this with the first phone call. So I phoned wandile back and asked him what must I do and he had no answer for me. I then ask to speak to the manager at his store @13:36 on the 09th. He said she’s on the phone. I then sent a what’s app message to their cell number at 15:15 to remind them to get the manager to phone me. Still nothing. At 09:18 this morning I again sent a message to remind them to get the manager to phone me. Yes u guessed it. Still nothing. At 13:15 I decided to phone. And spoke to wandile who then put me through to his manager. Sorry I was busy was her response. And then she proceeded to tell me that she spoke to the technician and the repair is not under warranty as the back of the screen is cracked. Okay so my questions are. U were so busy u couldn’t phone me back but u had time to speak to the technician? After speaking to him u still didn’t bother to pick the phone up? And how is it possible that the back of a screen can break but there is not a single scratch, chip or drop mark on my phone. Surely there should be some evidence that this phone was as the manager would say dropped. But there is absolutely nothing. How does a screen break when there is not other physical damage to my phone? Please could someone give me a response that justifies it because ‘it can happen’ does not sit well with me!