Accelerometers don't measure acceleration, they measure the difference between the force on a mass on springs and the body of your phone. If you drilled a tiny

Accelerometers don't measure acceleration, they measure the difference between the force on a mass on springs and the body of your phone. If you drilled a tiny hole and pulled the mass up, the sensor would think the phone was being pulled down. Acceleration due to gravity has an important property of acceleration in general: radiation. Spiraling black holes emit gravitational waves, in the same way that an accelerating electric charge emits light. There are free falling reference frames where uniform gravitational fields can go away, but the gravitation of a massive body isn't uniform and can't be eliminated by changing the coordinates. The relationship between gravity and fictitious forces is an important stepping stone, but it does not have all the properties of a fictious force, only some of them.