how to get the contact infomation by sms message

I'm working on an extend SMS application. And now i can read all the SMS message from the mmssms.db. In the SMS database table, the field 'person' indicate the '_id' in contact table. When 'person' is >= 1, that means the message is sent from people in the contact list. So i can 'managedQuery' from contact table. But the question is, in my mobile phone, the test program sometimes can NOT query the contact infomation even 'person'>= 1. So can somebody show me some correct ways to query contact infomation by 'person' filed in SMS table ?Here is some sample code which can make my question more clear:

class ContactItem {      public String mName;  }      ContactItem getContact(Activity activity, final SMSItem sms) {          if(sms.mPerson == 0) return null;          Cursor cur = activity.managedQuery(ContactsContract.Contacts.CONTENT_URI,                   new String[] {PhoneLookup.DISPLAY_NAME},   " _id=?",                  new String[] {String.valueOf(sms.mPerson)}, null);         if(cur != null &&             cur.moveToFirst()) {             int idx = cur.getColumnIndex(PhoneLookup.DISPLAY_NAME);             ContactItem item = new ContactItem();             item.mName = cur.getString(idx);             return item;         }         return null;     }