Updating Addressbook details

Hi All,
   We have a requirement to update the phone numbers in our customer master. Presently, we have extension to each number e.g., 123-456-7890 E 1234. Now we want to remove the extension as it is creating issues during shipping. But, when we try to update just the number using Webservices, it is updating all the other data (Address1, Address2, Zip etc.,) to NULL

We are using the following code
CustomerAddressbookList  custaddbooklist = new CustomerAddressbookList();
CustomerAddressbook[] custaddbook = new CustomerAddressbook[Addrec];
  for ( int kk = 0; kk<Addrec ; kk++)
    {
      custaddbook[kk] = new CustomerAddressbook();
      custaddbook[kk].phone  = "004-920-3281";
      custaddbooklist.addressbook = custaddbook;
     }
   customer.addressbookList = custaddbooklist;