I am unable to send/receive regular text messages on my iphone

I am unable to send/receive regular text messages on my iphone 3gs. All I can do is send/receive imessages. I've removed and replaced the SIM, erased all content on my phone and re-synced it, and made sure that it was updated, but nothing has worked. I was able to send/receive regular texts a couple of weeks ago. I don't have a lot of pictures on it, and I generally don't keep messages on the phone, so it doesn't get bogged down with too much data. ATT was unable to help me. Any ideas?