I got the solution
here is my code
ExchangeService service = new ExchangeService();
service.Credentials = new NetworkCredential(username, password);
service.AutodiscoverUrl(emailaddress);
Contact contact = new Contact(service);
contact.GivenName = "ABC";
contact.Surname = "XYZ";
contact.FileAsMapping = FileAsMapping.SurnameCommaGivenName;
contact.PhoneNumbers[PhoneNumberKey.HomePhone] = "";
contact.EmailAddresses[EmailAddressKey.EmailAddress1] = new EmailAddress("abc@yourdomain.com");
contact.Save(WellKnownFolderName.Contacts);