You may want to have your phone checked to make sure it's not a problem with it. Has this always been a problem or did it just start happening? Have you looked at the microphone are (bottom of the phone) to make sure it's not clogged or covered by anything causing sound to be muffled?
Thank you. I've had the phone for about 3 months and don't make calls very often. I usually text or email and use the data most frequently. I do have the phone in a case but there is an opening for the microphone. I'll take it to the Apple Store and have them look at it.