When searching for who to send to in a new message, it also comes up with results for the number based on those letters. Meaning, if I search "Kev" for Kevin, it will give me contacts not only with "Kev" in the name, but also if the phone number of the contact contains "538". I'm not sure why anyone would use letters to search for numbers in their contacts list. But this should at least be a configurable option for the search.