However, if you are implying that having a flight plan in the system will in some way ease your clearance into controlled airspace, this is only partially true. There is a considerable gap between filing a flight plan, and finally receiving a squawk, frequency and clearance to blast into controlled airspace. Your flightplan will not necessarily be sent to every conceivable class D provider that you might wish to use to organise you clearance, and therefore you will be treated with caution if you call xyz Approach up unannounced looking for asssitance.
One thing you can try, and seems to have a reasonable amount of success for me, is to add the address of the unit you wish to call first on the addressee part of your flight plan. In most cases, this is the ICAO id suffixed with 'ZTZX', but either the AIP or a phone call will give the definitive answer. (in fact, if calling the ATSU on the 'phone first, they might well tell you who they would like it addressed to)
Just another drop of oil into the vast number of cogs.....