The small speaker that you hold up to your ear when holding the iPhone is referred to in the users manual as the "Receiver."
My Volume on the iPhone is to low to use as a phone, period! I have let others check this and all agree that it is not going work for them if it is not fixed. The Apple Store rep told me he thought that my phone was to low to hear on. I did not have time to wait that day for an appointment for a genius, but I did call, I have a case number with Apple support and we shall see what happened from there. They were nice, and they have referred this to the engineers to see what if anything can be done. I am to hear back form them on my land line first of next week.
I have tried 3 phones now and all are too low when using as a phone when holding it up to my ear. This is a serious problem for me and I hope that it gets fixed. I wished this was just my phone or wax in my ear or plastic, or anything that does not have me returning the phone, but I am running out of ideas.