Mumbling, faint and echoing

When I call someone, they say I sound like I'm mumbling, faint of voice or echoing.  And I sometimes get my voice echoing back.  I'm already on my second iphone, thru the apple store, and they are saying that it's Verizon's network issue (something about 4G, volume of traffic and the location of the antenna in the phone).  Verizon's reply is to replace with a used phone from them or to upgrade and pay on a new phone plus the cost of new accessories.  Does anyone solution to this problem?  I don't want to have a fit in the store for a free solution, but I will!

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