Isn't it simpler just using an electronic tuner, they come pretty cheap these days, with the chromatic scale, which is handy for funny tunings.
But trying to tune to a mobile phone in a noisy pub sounds like hard work to me. I need thsat little green light or pointer on a dial.
And one handy thing with a tuner is that you can use it to check what key someone is singing in or playing in, and come in on target. Make people think you have perfect pitch, if you like. Can't do that with a mobile.