15 isn't an area code. When cellphones first came out here, in the early 90s, they had a special out-of-network treatment that required them to have a special dialing scheme. I believe it was because technically they could be assigned the same numbers as regular landlines, but someone might correct me on that.
So in the 90s the scheme was Country Code + Area Code + 15 + Cell number.
You no longer need the 15, just like you don't need to add a 0, but telcos still recognize it for legacy reasons.
Correct me if I’m wrong but if dialing from a landline you still need to dial 15.
Calls to a cellphone are still billed differently than landline calls, unless something has changed in the last 2 years. In Argentina, as in most countries, incoming calls to a mobile are billed to the caller - not the mobile number like in the US. Dialing 15 makes clear you know you’re calling a cell phone, and will be billed accordingly. (Most countries with such billing introduced separate area codes for mobiles; Argentina didn’t.) Yes, by now many if not most phone plans include unlimited calls to mobiles (just as in the US, most mobile plans include unlimited minutes), but the principle remains.
So the area code of most of Gran Buenos Aires is 11. It need not be dialed for a local call - but 15 must be dialed.
Calling domestically, 15 goes
after the area code, calling from abroad the 15 is replaced with 9
before the area code.
So calling from (a landline in) BsAs to a cell in, say, Cordoba: 0351
15 234-5678
From outside the country to that same number: +54
9 351 234-5678
From a landline in Córdoba to a BsAs mobile: 011
15 2345-6789
Same number from abroad: +54
9 11 2345-6789.
Because local calls omit the 11, and calls from mobiles omit the 15 as well, people conflate the 11 and the 15. But they’re completely different things. And calling from a landline in Mendoza, you need both.
Convoluted, but coherent once you’ve grasped it.
The number itself is usually 8 digits, but there are some very old numbers that still have 7 digits, from before the conversion.
I haven’t seen any actual 7-digit numbers in capital. I did see some written on signs, but they all need a 4 now.
Outside GBA, 3-digit area codes mean 7-digit numbers, and 4-digit area codes mean 6-digit numbers. Basically the area code+number always totals 10 digits.