Phone numbers stored in an address book are not guaranteed to be unique(at least in the US) because they can be stored with or without area code and with or without country code.
You can usually assume that the non-country/area-code numbers have the same country/area code as your current user, if you know their phone number (good thing to get during registration).
Hence the normalization step. You can probably guess the country code from other information (e.g. if your app is only in the US). If there's no area code included, then you can't match it up anyway, hashed or otherwise.