Anyway to decide which record types show up when doing a global search by default? We often talk to customers about items... so a ton of our Phone Calls have Item #s in them. I'm aware I can find only items by typing "i:item name" in the global search, but I was wondering if I could remove Phone Calls completely from the global search results

you would have to remove the users permission from Phone Calls to make them not show up in the global search. Or filter with the prefix as you mentioned.