title: How to check entered phone number is valid or not with flutter?
body: <p>I'm trying to check to entered phone number is valid or not .
  means, if I enter the wrong number which does not exist in the world, so it shows me a toast with content "Please enter a valid number" </p>
  
  <pre><code> Expanded(
     child: TextField(
       keyboardType: TextInputType.phone,
       decoration: InputDecoration(
         border: InputBorder.none, 
         hintText: "Phone Number", 
       ),
       onChanged: (value){
        setState(() {
          phoneValue=value; 
        });
       //String telNo = value==null?("+91" + value) :null;
       print("phoneNumbe:$phoneNo");
       this.phoneNo = isCountryCodeSelected ? "+" + countryCode + value : "+91" + value ;
       print("phoneNo="+phoneNo);
      },
     ),
   )
  </code></pre>