For local numbers(Your Country): Enter the phone number as if you were calling your friend on the phone.
For international numbers(Other than your country): Check to make sure the number is in the proper format with proper country codes.
When adding a contact's international number to your phone address book, start with a plus sign (+). Then, write the country code, followed by the full phone number.
Country Name: India
Country Code : 91
Phone Number: 9425746321
Phone Number should be saved as: +919425746321
Your friend's phone number must be saved in your phone's address book in order for you to start a chat with them in WhatsApp.
So, now just refresh contact list in WhatsApp and you can start a chat with these contacts.