Can I reserve over the phone?

go to recreation.gov you can reserve on line and they list their phone number to do it over the phone

Yes

Yes you can.. up to 6 months in advance...

Summer months are mostly reservation, during Spring time mostly first come first serve. You can reserve over the phone Goodluck

Yeah the resort takes reservations but I'm not sure about the campground... it's first come first serve in the off season