Can One Send A Text Message From A Computer To An AT&T Wireless (cell) Phone?

The 10 digit number then @mms.att.net

Now due to latest web technologies it is now possible to send text message from a website by using computer to any cellular network. One of the famous site where you can send sms & text messages is
 
www.sms.ac

Seems text from computer works for Verizon, Sprint, and some others, but not free for AT&T,