For the most part, you will likely be communicating via cell phone with other cell phones. In this case you don't need any area code. You just call the number that you have for that person. If you are calling a fixed phone, a house phone or business for example, that is outside the region you are in, then you'll need the area code:
City
Area code
City
Area code
Ayacucho
66
Lambayeque
74
Cerro de Pasco
63
Paita
73
Cusco
84
Puerto Maldonado
82