1. I'm on the phone.
('the' refers to a specific phone: the phone in the person's hand)
2. I'm on a phone.
(As opposed to, "I'm on a CB radio")
3. I'm on a cell phone.
(As opposed to, "I'm on a telephone; i.e., land phone.)
4. I'm on the cell phone.
(It's common to be more specific: "I'm on my cell, as opposed to, I'm on my house phone, office phone;i.e., land phones.)