Yeah. Call when line is short or non-existent. Better yet go to DiFara or Lucali, they are way way better. I'm sure you have heard the reasons. Grimaldi's is usually undercooked, doughy, the ingredients mediocre, and there is no love.
The phone number is 718-858-4300. They only answer the phone and do take-out orders when they aren't slammed. So if you're trying to call in and no one's answering, that's probably the reason. Grimaldi's is good - overrun by tourists - but good. I recommend DiFara's though - now THAT is amazing.
I haven't done it in about a year but I have ordered take out many times from Grimaldi's and never had a problem getting through between 5 and 6 in the evening. No secret number. Can you get through at any time? Have you verified you have the right number?