At least with a MoviStar SIM the dialing sequence from Argentina to International is:

+ countrycode areacode number [if your SIM is set up with the international dialing prefix)

or
00 countrycode areacode number

Dialing an Argentine cell phone from outside the country is a little wacky:

International access number 54 (for Argentina) 9 (for cell phones) areacode (11 for Buenos Aires) last 8 digits of cell phone number (ignore the 15 prefix)

John