Problem Summary. Over time, the iPhone collects dust and lint. This causes the microphone and speaker to become obstructed. The speaker and microphone are located on the bottom of the iPhone. They are covered and protected by a fine metal mesh screen. The small holes in the mesh screen can become clogged up with dust, lint, and fine debris. As a result, people will not be able to hear you as well on the phone.
Problem Solution. Use a firm toothbrush to loosen the debris, and while brushing the base of the phone, including the cable connection port, clean the area simultaneously with the hose tip of a powerful vacuum. This will loosen and remove the debris from the mesh grill. It should appear shiny and like new when done.