AWSCognitoIdentityUserAttributeType * phone = [AWSCognitoIdentityUserAttributeType new];
phone.name = @"phone_number";
//phone number must be prefixed by country code
phone.value = @"+15555555555";
AWSCognitoIdentityUserAttributeType * email = [AWSCognitoIdentityUserAttributeType new];
email.name = @"email";
email.value = @"email@mydomain.com";

//register the user
[[pool signUp:@"username" password:@"password" userAttributes:@[email,phone] validationData:nil] continueWithBlock:^id _Nullable(AWSTask<AWSCognitoIdentityUserPoolSignUpResponse *> * _Nonnull task) {
    dispatch_async(dispatch_get_main_queue(), ^{
        if(task.error){
            [[[UIAlertView alloc] initWithTitle:task.error.userInfo[@"__type"]
                                        message:task.error.userInfo[@"message"]
                                       delegate:self
                              cancelButtonTitle:@"Ok"
                              otherButtonTitles:nil] show];
        }else {
            AWSCognitoIdentityUserPoolSignUpResponse * response = task.result;
            if(!response.userConfirmed){
                //need to confirm user using user.confirmUser:
            }
        }});
    return nil;
}];