If you want to have the real name of the plane in the ATC, you haven't to put C172rg in the section
[General] atc_model=C172RG
but use the official ICAO code of the airplane. So, you have to put :
[General] atc_model=C72R
Keep atc_type=Cessna between General and atc_model.
So, you will have Cessna Cutlass insted Experimental. FSX use the ICAO codes to recognize plane until 2006 of course. If you have to use a new plane built after, you have to find a voicepack call sign and use Editvoicepack to put it in FSX database.
There is a website where you can find a lot of ICAO codes for your airplanes :
http://www.flugzeuginfo.net/table_accodes_en.php
Have fun
PS I'm sorry for my bad english, I am french.