The 11 and 15 mean completely different things. Here's the short of it.
11 is the area code for Greater Buenos Aires. (All other cities in the country have a 3 or 4-digit area code, but BA is just (0)11). When you call a phone in Buenos Aires, you don't have to dial the area code - just the 8-digit phone number. BUT, when you call a cell, you dial the prefix 15 to be aware that it is a cell phone you're calling, and will get charged accordingly. (When calling, a cell phone in a different city, you DO need to dial the area code, and then 15: if you're calling a cell in Concordia, you'll dial 0342-15-xxx-xxxx).
When you're calling from outside, the 15 gets replaced with 9. You're dialing 11 just to show that you're calling Buenos Aires; this is not taken for granted, as you're not placing the call from BA. So you're dialing 11 just to get to BA, even if you're not calling a cell: a call to a BA landline is +54-11-xxxx-xxxx. But a call to a cell in BA is +54-9-11-xxxx-xxxx.
Hope I've made it clearer!
P.S. I have movistar, as does my family. In my experience, when calling from abroad, even when dialing properly, your chances of getting through are not 100 percent; more like 50. If you dialed properly, and didn't get through, just rinse and repeat.
