Hi!
Check out http://www.sms-wap.com. I think they have what you are looking for. They have great pricing, you only pay per message.
You can send messages, ringtones, logos and so on. They also have two-way messaging, it means that the user send a message to their sms central and then a predefined script (for example php, cgi, asp) recieves the message and do whatever you want to. If the script outputs any information it will be sent back to the users phone.
Hope this helps!
/David
