Q: People sound muffled when I answer my phone

Hi-

I have been having issues lately where if someone calls me or I call them, when they talk to me it sounds very muffled and hard to understand them.  I have tried using a duster can that you use on computers on the ear piece and it helps for a little bit and then back to the same muffle the next day if I'm on the phone.  Can someone help?

thanks!