Local numbers follow a numbering plan called NPA-NXX-XXXX. NPA is the area code of the number. NXX is typically the rate center. As an example 438-648-XXXX and 438-688-XXXX both have the same area code 438. However they belong to two different ratecenters, 438-648 (Lachine) and 438-688 (Montréal). This is however not always the case. Some area codes such as 416 belong to one ratecenter (Toronto) and all NXX's of that area code belong to Toronto. Therefore as indicated above, it is important to ensure the ratecenter is portable.