Every now and then, when I unlock or lock my iPhone (meaning when I click the button to lock or unlock the screen, not when I make my iPhone usable on other networks) the sounds is off. It has a bit of an echo, sounding as though it's has a tin can over the speaker. Nothing is obstructing the speaker. This has happened a few times with calendar alarms and text message alerts as well. Anyone else experience this problem? It usually remedies itself after I place a call.