MAP and ISUP are different protocols.
ISUP is used to establish a TX path using a defined CIC (Time Slot
connection). MAP is used for Non-Circuit Related signalling.
For your case a request to the HLR is made via a SRI request.
So that HLRs can scale there is a SRF (signalling Relay Function) that
is generally a standalone node or integrated with the GMSC or the
STP Network. This allows HLRs to scale and removes the earlier issues of
batching MSISDN / IMSIs to a specific HLR.
MAP has been designed to use the MSISDN to the HLR. The HLR intern will
map the MSISDN to IMSI for Profile Information, Service Flags (CAMEL, SS, etc..),
Serving VLR, Location, etc..
For your case ISUP cannot be used as it have been designed to setup a “nailed up”
connection.