回答“Why do callers hear an echo during phone calls?”I feel foolish ... after having tried everything, but really everything conceivable but could not use WhatsApp, Viber etc. when using the Speaker phone. Normal phone calls were ok. I finally looked for the small mike hole on top - after all, many of you recommend to cover it with a small piece of plastic - and then found that I had not properly removed the protective (almost invisible) plastic cover that the supplier had placed there to protect before shipment... I removed it and now have a S4 the way Samsung designed it. Works perfectly. For the first time in a long time!