I am a fan of calling relative to beginners. However I do not think that that is the norm in the US (please correct me if I am mistaken)
Mac- You are correct. At least for the half hour call, it's not the norm to make the call relative to beginners or places. The main reason for this is that with union casts, their standard call time is 30 minutes before the top of the show, not 30 minutes before places. As far as 15 min, 5 min, I do call those relative to the places call, and make the places call between 2 and 5 minutes before GO depending on where the dressing rooms are (usually more like 2 minutes)
The theatre I work at, I need to call the show name but not the company. Since there is one paging system serving 2 different stages and usually 2 different casts, it's necessary to denote who I'm talking to---however, since the paging system is shared, many times an ASM from one of the casts will make both calls simultaneously, as the same information is going out... as in "Ladies and Gentleman of X and Y, this is your 15 minute call. 15 minutes until the top of Act I."