Pls. remember several things. If I'd seen this post earlier (closer to your first day) other than suggesting you calm down (breathing deeply helps accomplish this) I'd suggest listening closely & maybe even taking some notes. Sometimes it's not about asking questions, but rather abt listening closely to what's said.
As to the callers & a script for that: this is important, while it's necessary to alway remain calm & polite, one's obligation is to the Boss FIRST, not the caller.
Don't say the Boss is busy (that's obvious).
Say something like: I'm sorry but Mr. Smith is not available. My name is Karen, I'd be happy to take your name and number (always read back the number, showing them you have it correctly, ask them to spell their name if it's not a common one or you're unsure) & importantly, make sure to get info on WHAT THEY WANT (why they're calling).
Employers don't like just a message of Jane Smith called, that's not useful, especially if they don't recognize the caller's name.
They need to know on the very first message WHAT THE CALLER WANTS. This helps them to prioritize the response (how urgent is it) & also to determine if someone else can handle it other than the Boss.
Far more useful is something like:
Jane Smith called (tel # xxx-yyyy), she needs to reschedule your apppointment with her for later this week; while I've told her I'd have to check your scheduling with you, I've suggested that alternate dates for you would likely be best on 3/24 & 3/26 as those are the only days you have any openings right now.
So if this example were true, the Boss could take the info. I gave, confirm if the dates I offered were still good & then ask me to call the caller back & reschedule the appointment based on those dates.
Initially, it may be difficult to deal w/ anxious callers, just pls. remember that it's your office, not theirs. Don't let them intimidate you, politely but firmly stand your ground.