I'm generally follow the same pattern as Matthew.
Half hour is called at 30 minutes to top of show, and I will announce it as "Attention ladies and gentlemen, this is your half hour call. We are at 30 minutes to the top of the show" and include anything else that needs to be included (reminder to sign-in, any company announcements, when stage management will be collecting valuables, etc).
All other calls are announced based on when I expect to call places and will be announced as "x#of minutes to places" whether that be 2, 3, or, on a very rare occasion, 5 minutes before the start time (I did have to do a places call at 5 minutes to start once on a very large opera as almost the entire 60 member chorus entered right at the top of the overture and we needed that much time to count heads and make sure props were all distributed correctly).