Calls break up and sound like a robot.
Since I bought a second hand 8 plus my calls have been garbled (robotic) every third or fourth call . I was with Virgin (O2) and thought the problem lay with them but I recently moved to Lebara (Vodafone) and the problem is still there. The call starts well but within 30 seconds the voice becomes robotic for another 30 seconds before becoming clear again. After another 30 seconds it breaks up again. The fault seems to be with the phone rather than the telecom company; I have cleaned the speakers and microphone and followed other adivce (WiFi Call, Airplane mode etc.). Signal strength is always good.
iPhone 8 Plus