Removes all characters except those that are allowed in a telephone number

NSString+PhoneNumber.m

-(NSString *)cleanPhoneNumber
{
    if (self.length == 0)
        return nil;

    NSMutableCharacterSet *mutableCharacterSetForCleanPhoneNumber = [NSMutableCharacterSet new];
    [mutableCharacterSetForCleanPhoneNumber addCharactersInString:@"+0123456789"];
    [mutableCharacterSetForCleanPhoneNumber invert];

    return [[self componentsSeparatedByCharactersInSet:mutableCharacterSetForCleanPhoneNumber] componentsJoinedByString:@""];
}