No they are not, because there's a clear boundary between your own device and someone else's infrastructure. And you could as well build your own landline syste

No they are not, because there's a clear boundary between your own device and someone else's infrastructure. And you could as well build your own landline system — which is exactly what people do in commercial buildings. You could also build some sort of scrambler device that goes between the phone and the line. It's the same as asking "is internet backdoored". It might be wiretapped, but you can use encryption to avoid that risk.