Hi cat & scot
MSC/VLR send the paging to the whole LA which MS is in.(IMSI or TMSI),better by TMSI cuz it has small size (4TMSI in one paging block)
IF no responce, MSC/VLR send 2th paging to the whole MSC area. better with IMSI (2IMSI in paging block). cuz MS may just crossing to new LA & its TMSI on VLR not change yet, or my be it powered on or connected to network in diff area & its TMSI not changed correctely, so cuz of all this the 2th paging must be with IMSI.
furthermore there is two stage timer in MSC,one to trigger the 2th page after defined time from the 1st page, and other stage is expire timer to drop the call after its time from 2th page.