When you sync a phone with your computer using iSync 2 (part of Mac OS X 10.4 Tiger), any Address Book phone number that contains uppercase (capital) letters will be automatically converted into all numbers on your phone.
For example, if you have the phone number 800-555-CALL in Address Book and then use iSync 2 to sync contacts with your phone, the phone number will show up as 800-555-2255 when viewed on your phone.
iSync translates uppercase letters into numerals so that the dialing feature works with some phones. Please note that lowercase letters are not translated because some phones use lowercase letters for commands (for example, the letter p is used to pause on some phones).