Not all services support short codes.
I am currently using Callcentric. They do not officially support them. They clearly stated this when they started offering the service.
But I found that while I could not send messages to short codes, I did receive a number of messages from them over Callcentric SMS services.
That is a good thing since a lot of Web sites (especially financial sites) are using two factor authentication for security. The text messages they send for this purpose come from short codes.
Example: When logging onto a Web site, they text a message that gives you an authentication code to enter on the Web site to complete your log in. They send you this after entering your user name and password [so you can enter it into a subsequent login prompt on your Web site]. Most of you have experienced this.
When I previously tried to send messages to short codes, they would bounce.
But I just tried tweeting one to [short code removed - bh]. It actually replied to me. So perhaps they've since implemented it and not said anything about it.
Regards,
Fred
+--------------------------------------------------------------+ Who is John Galt?