Can't figure out why I'm getting an "undefined name" error in my custom action

Undefined name 'authManager'. Try correcting the name to one that is defined, or defining the name.

I'm just messing around with getting phone authentication working via a custom action, but can't figure out why there's an error about authManager, when it's defined in

../../auth/firebase_auth/auth_util.dart'

(I'm new to Flutter/Dart, so pardon my ignorance if it's a simple Dart issue.)

// Automatic FlutterFlow imports
import '/backend/backend.dart';
import '/backend/schema/structs/index.dart';
import '/actions/actions.dart' as action_blocks;
import '/flutter_flow/flutter_flow_theme.dart';
import '/flutter_flow/flutter_flow_util.dart';
import '/custom_code/actions/index.dart'; // Imports other custom actions
import '/flutter_flow/custom_functions.dart'; // Imports custom functions
import 'package:flutter/material.dart';
// Begin custom action code
// DO NOT REMOVE OR MODIFY THE CODE ABOVE!

import '../../auth/firebase_auth/auth_util.dart';

Future customFirebasePhoneAuth(BuildContext context,
    String screenNameToNavigate, String phoneNumberDigits) async {
  print('customFirebasePhoneAuth called');

  final phoneNumberVal = '+1${phoneNumberDigits}';

  final authReturn = await authManager.beginPhoneAuth(
    context: context,
    phoneNumber: phoneNumberVal,
    onCodeSent: (context) async {
      print('inside onCodeSent');
    },
  );

  print('authReturn: $authReturn');

  return authReturn;
}

Thanks for any help!