1. The person is using a phone to type instead of a computer, as an excuse for typing slowly or typing with horrible grammar. Commonly used angrily and/or childishly.
2. The person is using a phone and wants you to getaway.
1. "You type really slow," I typed. "Im on a phone dumbass," they replied.
2. "My kid needs milk," I said. "Im on a phone dumbass," they replied, putting their phone up to my face.