Text message server

can anyone explain how those text message services work, that we see on MTV.
Clarification:
The commercials that ask you to text message 5555 with the word hungry, to get a certain ring tone, sent back to your phone

I understand that all cell phone numbers have email address assigned to them, but how do they assign 5555 to whatever server they are using…

Hope the question is clear
thanks fo the help…any other info on the subject would be great