How to append phone number to an existing contact.

    //We need to create a mutable copy of ABMultiValueRef with property kABPersonPhoneProperty

    

    ABMultiValueRef Phones = ABRecordCopyValue(contact, kABPersonPhoneProperty);

    ABMutableMultiValueRef Phone = ABMultiValueCreateMutableCopy(Phones);

    ABMultiValueAddValueAndLabel(Phone,@"413-675-1234", kABOtherLabel, NULL);

    ABMultiValueAddValueAndLabel(Phone, @"413-675-1235", kABOtherLabel, NULL); 

    ABRecordSetValue(contact, kABPersonPhoneProperty, Phone,nil);

    CFRelease(Phone);

    //This will add 2 numbers with others title under phone section of contact.

    //You can use other label too like following kABWorkLabel or kABPersonPhoneMobileLabel 

    ABMultiValueAddValueAndLabel(Phone,@"413-675-1234", kABPersonPhoneMobileLabel, NULL);

    ABMultiValueAddValueAndLabel(Phone, @"413-675-1235", kABWorkLabel, NULL); 

    //Dont forget to save your addressbook after that.

    ABAddressBookSave(addressbook, &error);

    if (error != NULL) 

    {

        NSLog(@"failed");

    }

      NSLog(@"success.");

Note: If you don't make a  mutable copy, you will get following error for trying add  phone number to ABMultiValueRef instead of ABMutableMultiValueRef. As you created ABMultiValueRef by copying from ABRecordRef using method ABRecordCopyValue.

Assertion failed: (((ABCMultiValue *)multiValue)->flags.isMutable), function ABMultiValueAddValueAndLabel