A base hospital is, like previous poster mentioned, a hospital designated by the EMSA (or Emergency Medical Services Authority) to give EMS personnel orders for treatment of patients in the field and designate the nearest appropriate destination hospital for said patients depending on certain criteria. Contrary to previous poster, the base hospital does not only designate destination for MCIs (or mass casualty incidents), they designate destination for ALL calls. For example, if EMS calls a base station and a patient exhibits stroke-like symptoms, the MICN will direct them to the nearest primary stroke center open to receiving patients. A non-base hospital is any and every other hospital that is not designated to receive EMS calls.
The format/procedure for answering the phone is irrelevant to you right now, and will be for quite sometime actually, since in LA County you have to have at least one year of RN experience before being qualified to start MICN training. At my facility, nurses typically don't get offered to get MICN trained until they're 3 years in and have more than ample ED/trauma/triage experience. Even if your facility only requires the year as required by LA County, that's a year more ED experience and knowledge you'll have than you have now. I love your foresight and initiative, but as a nursing student, I would focus less on what the MICN does and more on what the other ED nurses do since that's what's you'll be doing right out of school if you decide to work in ED.