Yep, we use Skype to call locally everyday. I have a Skype Unlimited World and I use it to call everywhere including back to US and here locally in BsAs. I think it was $10 a month when I got it, not sure.
Only thing is, this does not cover calls to cell phones here in Argentina. You'll have to add Skype credit for that.
It's easy to make local calls with any Skype account.
Click "call phones" to open a new call box.
Choose "Argentina" from the dropdown list. This automatically dials the +54 for you.
Dial 9 11 and then the 8 digit number (note that the 11 replaces the 15 you might otherwise use for a cell phone).
There is a whole Skype message board dedicated to this topic here.
I've found that it's easy and worth it to call landlines. I've had difficulty in the past with calling cell phones (and I think it drains the person's cell phone you call at the same time). But with the instructions above, I'll try-try again.
Yes, the person that calls from a cell phone uses up their credit. Bur if you call a cell phone from skype or a locutorio you will be consuming the person's credit. Only calling from a landline doesn't use up the person's credit.