1. I'm
on the phone. ('on the phone' is this a stock phrase? Why does a lot of people say this? I don't even know which phone you are on when they say this.)
2. I'm
on a phone. (How come no a lot of people say this?)
3. I'm
on a cell phone. ('on a cell phone' is this a stock phrase? I googled it and more people say this then the one below.)
4. I'm
on the cell phone. (How come not a lot of people say this?)