Tell them your availability.
Be yourself expect to work hard
Yes definitely agree with putting a set day of no availability. I thought i would get days off just different each week. But they will make good use of your availability if they need you.
The questions are generally how would you approach a situation relevant to this job. Nothing too difficult really.
Run for the hills
Do not set full availability and make sure you you say you're unavailable for atleast one day during the week. Be forward and assertive with them, as asking once or not asking aggressively enough yields no results. Write your calls and call durations down.