This is what i found in spec
GSM 03.12 [ Location Registration Procedure ]
188.8.131.52 Downloading of subscriber parameters to the VLR
As a part of the location updating procedure, the home location register will convey the subscriber
parameters of the MS which need to be known by the visitor location register for proper call handling. This
procedure is also used whenever there is a change in the subscriber parameters that need to be
conveyed to the VLR (e.g. change in subscription, a change in supplementary services activation status).
If the HPLMN applies the multinumbering option, different MSISDNs are allocated for different Basic
Services (see GSM 09.07) and stored in the HLR. Among these MSISDNs, the Basic MSISDN Indicator
as part of the HLR subscriber data (see GSM 03.08) marks the ‘Basic MSISDN’ to be sent to the VLR at
location update. It is used in the VLR for call handling as calling party and as line identity.
But i of what i can understand and analyse this is not “for sure” access method to collect MSISDN given IMSI.