Im not certain of much but I am certain they aren't paying that much for phones:) So def not DID's

Basically they have 3 numbers that can be dialed into from the outside. Example (555)555-5555 rings to CO1, (555)555-5556 to CO2 and (555)555-5557 to CO3. Any idea how I would assign these numbers to the CO's using keystrokes on the phone? Or what that might be called in the manualso I could look it up??