We stayed at the Sheraton Libertador in May, 2007. I have an unlocked phone with T-mobile service, but it matters only the the phone be gsm and unlocked. Argentina uses the same frequency as T-mobile and AT&T here in Norte America. Florida Street is one block from the hotel. Out the front door, right, right, when you reach Galleria Pacific this is Florida, turn left exactly one block and cross the street. The CTI Movil office is on the corner.

Go to the first floor and ask to buy a SIM. They will either sell you one, or send you upstairs to get one. Activation is done upstairs, (elevator available) and takes approximately one hour. Go shopping at Galleria Pacifica or if you do not want to walk the one block, stop off at a leather shop called "Maybe" and get measured for a leather coat. Maybe is located about half way down the block on the other side of Florida and English is spoken.
When you return to CTI Movil, buy a top-up card and go upstairs to get you activation.

Or, PM and I have a SIM that is probably still able to be topped up.