iPhone 4S Messaging: Can't send or receive non-iPhone messages

I can't send or receive messages on my new iPhone 4S. After activation difficulties, I can not send texts or photos, nor can I receive. Sent images to two people with HTC phones, they came back "Message Send Failure"; text messages appear to have gone through (no error message), but they apparently are not received (checked their phones and there aren't any texts from me). HOWEVER, I did receive texts and images from a friend who has an iPhone 3S - not sure if he got my replies.

Anyone else?? Driving me CRAZY!