Let's not overthink this one.
Under VFR, make the call, you can't talk on the radio and on the phone at the same time anyway. I can garantee you that a cellphone does not effect a pitot static instrument such as altitude and airspeed. You can hold heading by looking outside. Make the call, done it numerous times.
The only restriction is your cell phone provider, my current one is not much good above 3000' AGL.
Under IFR ( and IMC) you shouldn't have time to chat on the phone with your buddies anyway
so unless it's an dire emergency I wouldn't do it.
If I have a total electrical failure I'll use my phone since there is nothing left to influence anyways...
