How can I fix iPhone echo on speakerphone calls?

I can't say if this happens when using other apps such as WhatsApp. I only use the phone function of the iPhone for calls. Unfortunately I cannot use speakerphone because the person on the other end of the line experiences a very annoying echo. I have taken the phone in to have the speaker and microphone cleaned and that still did not resolve the issue. I was about to give in and buy another phone, but I've seen where other people have tried this just to have the problem follow them to the new phone. That leads me to believe there is something attached to my profile settings that is causing this. I'm looking for suggestions on adjustments that can be made in the settings.

[Re-Titled by Moderator]

Original Title: If I use speakerphone, the person on the other end experiences an echo.