If the AIP says you can enter the airspace with permission, it must give you details of how to obtain that permission. If its by phone you will need to obtain it prior to the flight. If its an airborne clearance then you need to contact them prior to entry and they may well specify a minimum warning period. There could well be delays as they probably don't man the frequency continuously.
You always tell the ground station when you change frequency otherwise they could notify you as missing!
A FISO will not tell you to do anything, they only provide information.
A MATZ has a specific set of rules, a Restricted Area will have its own published rules, they may or may not be similar.