Once your cell phone, or any GPS enabled device, detects itself reaching a speed greater than 1,000 knots (1,200 MPH) at an altitude of 60,000 feet or higher it is required to automatically disable itself.
This is due to a government regulation created to prevent GPS technology being used to guide intercontinental ballistic missiles.
During the Cold War era the United States and 16 other nations formed the Coordinating Committee on Export Controls (CoCom). One decision CoCom made that is still in effect is that all GPS units must automatically disable themselves as a security precaution when their speed and altitude could be indicative of a missile.
This is known as the CoCom limit. Some GPS manufacturers program devices to disable themselves when both limits are reached as the regulation calls for, while others program their devices to disable when either the height or speed limit is reached.
For most legitimate GPS uses it doesn’t seem like this would matter. However there are hobbyists who run into CoCom limit problems when attempting to implement GPS technology in things like high-altitude balloons.
It’s not that their balloons travel so fast, but that some manufacturers will disable the GPS when the altitude limit is reached even if the speed limit isn’t reached (as mentioned above).