iPhone 6 and 6S Speakerphone with WiFi Calling
 

Happened with my old iPhone 6 and my new iPhone 6S so really not thinking it is hardware. When at home, on WiFi calling (regular signal here not so great) and talking on Speakerphone, at random intervals the speakerphone will just shut off and revert to the regular speaker. Annoying. ...

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Courtesy of WestNet Wireless
www.westnet.ca