How to Save a phone Number in Windows Phone 7 using C# ?

The Windows Phone 7 API exposes a launcher ( SavePhoneNumberTask ) that will launch the contacts and enable to save the provided phone number to the contacts list .

To use this launcher , we might include the namespace Microsoft.Phone.Tasks.

using Microsoft.Phone.Tasks;

The SavePhoneNumberTask  chooser has a property PhoneNumber which can be used to set the phone number and the show method to launch the contacts application .
You should make sure that you set the phone number before you call the show method . 

private void PhoneApplicationPage_Loaded(object sender, RoutedEventArgs e)
{
     SavePhoneNumberTask savePhoneNumber = new SavePhoneNumberTask();

     savePhoneNumber.Completed += new EventHandler<TaskEventArgs>(savePhoneNumber_Completed);

     savePhoneNumber.PhoneNumber = "1987654320";

     savePhoneNumber.Show();
}

void savePhoneNumber_Completed(object sender, TaskEventArgs e)
{

     MessageBox.Show("Phone Number Saved");

}

When the savePhoneNumber.Show() method is called , the Contacts screen in WP7 is shown which enables the user to add the Phone Number to a new Contact or include the phone number to the existing user .

Once the phone number is saved , the user should press the back key to come back to the App .

Also note that similar to ComposeSMSTask , the current API doesn’t support saving the contact without opening the Contact Screen .