Why does my iPhone echo when I get a call from a landline?

Episode 1143 (1:56:16)

Paul from Pasadena, CA
Telephone

Paul just upgraded to an Apple iPhone 6 Plus, but every time he gets a call from a landline, he gets an echo. Could it be the Wi-Fi calling feature or noise suppression? Could it be a T-Mobile issue? Leo doesn't think so. But it could be a delay when talking from Wi-Fi or cellular to the landline. This is an odd issue that often happens on the radio show. And they've never been able to track it down. Leo says to get Apple to replace the iPhone. If it still happens, it's the network, not the phone.