Your phone will probably annoy a few pilots and air traffic controllers. But, most likely, not badly enough for them to take action against you, if that’s what you want to know.
You may have heard that unpleasant noise from an audio system that occasionally happens when a mobile phone is nearby A phone’s radio emissions can be very strong, up to 8W; they cause this noise due to parasitic demodulation.
you can actually hear such noise on the radio while flying. It is not safety critical, but is annoying for sure.
Of course, there is plenty of attenuation between phones in the cabin and the pilots’ radio. However, if say 50 people on board are inconsiderate enough who can’t be bothered to switch their cell radio off, there will be 50 phones constantly looking for cell towers at maximum power. That is a lot of radio pollution.
When inflight cellular service is provided, there is a cell station right beside those phones. They communicate at very low power without causing any disturbance.
Wi-Fi signal is much weaker (100mW) than GSM at its peak, and I never heard of it causing any problems.
Basically, having a bunch of cell phones moving along at 500-600 mi/hr, constantly trying to connect and disconnect from ground-based cell towers many miles below them interferes with cell networks. Most cell networks* weren’t designed for handsets moving at such a high speed and so far above the Earth’s surface. It can apparently degrade the networks significantly
Your battery goes dead, very quickly. I’ve done this several times by accident, and what happens is your phone ramps up its transmission power trying to find a tower (which it can’t do, because there are no cell towers at 30,000 feet, unless there’s one on the plane itself, of course), draining the phone’s battery until it runs out of juice and croaks.
Some older GSM phones would instead crash or hang during the ascent phase of the flight. During this phase the phone will likely receive many tower beacons (mostly distant ones on the horizon; the radiation patterns of most cell towers are arranged so that little signal goes upwards, and so towers that are below you will not be visible to the phone, but towers on the distant horizon will be). Some older GSM phones had firmware that could not handle a large number of towers all beaconing at the same time and would crash or lock up should this happen.