import 'package:flutter/material.dart';
import 'package:flutter/services.dart';

void main() => runApp(App());

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: SafeArea(
          child: TextField(
            controller: TextEditingController()..text = '123',
            inputFormatters: [SomeFormatter()],
            keyboardType: TextInputType.phone,
          ),
        ),
      ),
    );
  }
}

class SomeFormatter extends TextInputFormatter {
  @override
  TextEditingValue formatEditUpdate(TextEditingValue oldValue, TextEditingValue newValue) {
    print(oldValue);
    return newValue;
  }
}