Hmm, I made a test, added 4 contacts with the same name, then tried the dial box and worked fine...so, how could I replicate the issue? Are you sure you have the phone field populated in all entries? IF there is no phone number, then entry will be skipped from the dial box search.
If you want to look at code, the vphonebook.php file is the one that performs the searches for the Dial box field.