Motorola 1000 old type GSM, 8 Watt power. A few years ago: Rhein lost our Transponder signal when we passed thru FL 280 in the desend. We had the phone shortly on and then immidiatly off again. It caused a MD11 climbing as well. So trust it, a mobile has an affect. If we would know exactly what effect then the mobile phone company´s would love to eliminate that. Can you immagine how many roaming charges they could make in the air? Keep it off and there is a good reason for it. Accept the fact and don´t debate.
Fly safe and land happy
NG