WTA requires a WTA User Agent on the cell phone and a WTA server that resides somewhere on the telephone network. The WAP documents don't specify how the WTA server interacts with the phone network, since that varies depending on the phone system you use. This makes senseHTTP doesn't specify how a Web server retrieves files. It only notes that the client can ask for a file and receive it. The same thing holds true for WTA. It specifies that you can dial a number, for example, but it isn't concerned with how that actually occurs.
To simplify interactions with WTA, the specification calls for a WTA library that handles common tasks for applications. Applications use a super set of WML known as WTA-WML. These documents start with the wtai:// protocol identifier.
The phone has storage for some WTA-WML code in a place known as the repository. The repository stores resources (scripts, usually) in groups called channels. You can assign channels to particular events. For example, you might want some script to run when the phone receives an incoming call. Another channel might notify you that e-mail or voice mail is waiting for pick-up.
Of course, you don't want a WML script dialing a 900 number and running up your phone bill. To prevent this, WTA has a robust security feature that lets you trust some applications more than others.
More information is available online in the WTA specification at www1.wapforum.org/tech/documents/WAP-169-WTA-20000707-a.pdf. Although the specification is a bit dry, the example applications in Annex A are quite informative.