Incoming calls to a Mobile Station:

The call goes from the PSTN to a Gateway MSC (GMSC) belonging to the
subscriber's mobile carrier.

The GMSC uses MAP to query the appropriate HLR (based on the
subscriber's MSISDN) for the subscribers current location (VLR).

The GMSC then  sends a MAP query to that VLR requesting a MSRN
(temporary routing number).  The MSRN is used to route the call from
the GMSC through the PSTN to the serving MSC/VLR. (e.g. using ISUP)

The GMSC then sets up a call to that MSRN.  The serving MSC/VLR will
then start paging the mobile station associated with the MSRN.  The
MSRN is released back into the pool for other incoming calls.

Voila!