Firebase Flutter phone authentication verificationCompleted never get called

Question

i'm would like to add a button for existing user to verify phone number on their profile page,

   Future<void> phoneVerification(String _mobile) async {
    await _auth.verifyPhoneNumber(
      phoneNumber: _mobile,
      verificationCompleted: (PhoneAuthCredential phoneAuthCredential) async {
        await userInfo.linkWithCredential(phoneAuthCredential);
      },

      verificationFailed: (FirebaseAuthException e) async {
        // Handle error
        print(e);
      },

      codeSent: (String verificationId, int resendToken) async {
      },

      codeAutoRetrievalTimeout: (String verificationId) async {
      },
    );
  }

however nothing happened even i entered the OTP, verificationCompleted is not called

FlatButton(
          onPressed: () async {
            setState(() {
              PhoneAuthCredential phoneAuthCredential = PhoneAuthProvider.credential(verificationId: verificationId, smsCode: OTP);
            });
          },
          child: Text("VERIFY"),
          color: Colors.blue,
          textColor: Colors.white,
        ),