Use a generic address, like Dear Sir or Madam, and write the letter with the best content possible. If its good, it will be helpful. You can do your best to try to find out who will read the letter, and try to address personally. If you'd met the person at some sort of open house, or career event, or you'd heard them speak, that's fine.
If I understand your "wink" icon, you seem to imply that you will call, pretending to be someone else, to get their name. That's ... that's just wrong ... improper behavior ...bad. The concept just leaves me flustered. Dishonest. Needy. Borderline stalking. Seriously, just try to clear your mind a do a good job on the letter's content.