Then you've got a wrong number.
To dial a cell phone number in any area code from a landline or cell phone, you dial area code+15+local number. For numbers in Capital, you dial 011-15-xxxx-xxxx. You can skip the 011 because you're in the same area code.
To dial a cell phone number in Mar del Plata, you dial 0223-15-xxx-xxxx (the local part is 7 digits). A cell phone in Pilar would be 02322-15-xx-xxxx, a cell phone in La Plata would be 0221-15-xxx-xxxx.
If you're calling from a cell phone, you actually don't need to dial 15 at all.