Pathetic service from company

Did an upgrade on my business contract. Agent told me the phone (samsung s8+)was on the way to me only to find out a couple of weeks on that they didn't have stock and I was waiting for nothing. Eventually they manager agreed to give me an alterntive (iphone 8 256gb) on arrival I received a 64gb and the device is battery is not lasting. Trying to get hold of someone to resolve this issue but nobody is willing to return my calls. The 7 day exchange period will be up soon. How do I reslove this issue?