1. Head over to the Firebase Console.
2. Either start a new project or pick one you already have.
3. On the left sidebar, click on "Authentication."
4. Switch to the "Sign-in method" tab.
5. Turn on the "Phone" sign-in method.

1. Choose your platform (iOS, Android, or Web) and follow the steps to integrate Firebase into your app.
2. For Android or iOS, grab the google-services.json or GoogleService-Info.plist and add it to your project.
3. For web, copy the Firebase config and pop it into your app’s initialization code.

1. For Android:

• Add the Firebase Authentication dependency to your build.gradle file:
  ```gradle
  implementation 'com.google.firebase:firebase-auth:xx.x.x'
  ```

1. For iOS:

• Add Firebase Authentication via CocoaPods. Update the Podfile:
  ```ruby
  pod 'Firebase/Auth'
  ```
• Run pod install.

1. For Web:

• Include the Firebase Authentication module:
  ```javascript ```

1. Android Implementation:

• Initialize Firebase Auth:
  ```java
  FirebaseAuth mAuth = FirebaseAuth.getInstance();
  ```
• Set up phone number verification:
  ```java
  PhoneAuthOptions options =
  PhoneAuthOptions.newBuilder(mAuth)
  .setPhoneNumber(phoneNumber) // Phone number to verify
  .setTimeout(60L, TimeUnit.SECONDS) // Timeout duration
  .setActivity(your_activity) // Activity (for callback binding)
  .setCallbacks(callbacks) // OnVerificationStateChangedCallbacks
  .build();
  PhoneAuthProvider.verifyPhoneNumber(options);
  ```
• Handle callbacks:
  ```java
  PhoneAuthProvider.OnVerificationStateChangedCallbacks callbacks =
  new PhoneAuthProvider.OnVerificationStateChangedCallbacks() {
  @Override
  public void onVerificationCompleted(PhoneAuthCredential credential) {
  signInWithPhoneAuthCredential(credential);
  }
  
  @Override
  public void onVerificationFailed(FirebaseException e) {
  // Handle failure
  }
  
  @Override
  public void onCodeSent(String verificationId,
  PhoneAuthProvider.ForceResendingToken token) {
  // Save the verification Id and resending token
  }
  };
  ```

1. iOS Implementation:

• Initialize Firebase Auth:
  ```swift
  let auth = Auth.auth()
  ```
• Set up phone number verification:
  ```swift
  auth.verifyPhoneNumber(phoneNumber, uiDelegate: nil) { (verificationID, error) in
  if let error = error {
  print("Error: (error.localizedDescription)")
  return
  }
  UserDefaults.standard.set(verificationID, forKey: "authVerificationID")
  }
  ```
• Sign in with verification code:
  ```swift
  let verificationID = UserDefaults.standard.string(forKey: "authVerificationID")
  let credential = PhoneAuthProvider.provider().credential(
  withVerificationID: verificationID!,
  verificationCode: verificationCode
  )
  auth.signIn(with: credential) { (authResult, error) in
  if let error = error {
  print("Error: (error.localizedDescription)")
  return
  }
  // User signed in successfully
  }
  ```

1. Web Implementation:

• Import and initialize Firebase Auth:
  ```javascript
  import { getAuth, RecaptchaVerifier, signInWithPhoneNumber } from "firebase/auth";

  const auth = getAuth();
  ```

• Set up reCAPTCHA and send verification code:
  ```javascript
  window.recaptchaVerifier = new RecaptchaVerifier('recaptcha-container', {
  'size': 'invisible',
  'callback': (response) => {
  // reCAPTCHA solved, allow sign-in with phone number
  }
  }, auth);

  signInWithPhoneNumber(auth, phoneNumber, window.recaptchaVerifier)
  .then((confirmationResult) => {
  window.confirmationResult = confirmationResult;
  }).catch((error) => {
  console.error("Error during phone number sign-in", error);
  });
  ```

• Verify the code sent to the phone:
  ```javascript
  const code = prompt('Enter the verification code sent to your phone');
  confirmationResult.confirm(code)
  .then((result) => {
  const user = result.user;
  console.log("User signed in successfully", user);
  }).catch((error) => {
  console.error("Error during confirmation", error);
  });
  ```

1. Make sure your app can handle authentication errors and edge cases smoothly.
2. Adjust security rules in Firebase to ensure proper access control.
3. Add extra safeguards if needed, like using Firebase Functions to log attempts or flag suspicious activities.

By following these steps, phone number authentication can be successfully implemented in a Firebase project.