My blackjack can send, but not receuve text messages... ?

I just got a blackjack today. I sent a text to my sister just to make sure it worked or whatever. she got the text and replied. but I got no reply. I've restarted the phone many times. I don't know what to do really do other than that. I'm completely new to windows mobile. and apparently, cingular customer care is closed now. I'm only going to be using the phone for a couple weeks, on a count of mine is broken. but I'd still like it to work... any suggestions on what might be wrong?