How to make asterisk system automically response SIP call?

My objective: I want to use softphone(3CX phone) register with asterisk server, and make call to the server and asterisk act as a server to automatically response something, like play a song.

How i did: I installed asteriskNow using virtualbox, and registered the softphone by setting exntension for my SIP device(extension 333). And i write a dialplan in etc/asterisk/extensions.conf. The dialplan is :

exten =>s,1,Answer()
exten =>s,n,Playback(dir-intro-oper)
exten =>s,n,Hangup()

I want any incoming call to server, the server will automatically answer, and play a pre-defined voice (dir-intro-oper.gsm ) then handup.

But I met the problem is:

I use softphone, and i dont know which number i should dial to the asterisk server. Should i set up a extension number for asterisk server itself? If so, how to do that? By setting up SIP truck? Write the dialplan in sip.conf? or anything else?

Another questions:
I read the asterisk related book<<asterisk, the future telephony>> which tell us to write dialplan in the extensions.conf directly, but i found the extensions.conf in the server which alerts us do not modified the file directly, must use web-gui to modify.So which way i should follow?

In this case, i do not use any other hardware phone.
I am a novice on asterisk, please give me some hints and detail procedure.

Make sure the phone is in the incoming context (sip.conf (GUIs may use users.conf)).

Replace s by the required extension number. Dilal that number from the phone.

The Hangup is redundant after Asterisk 1.2. The Answer is also redundant, although sometimes an Answer followed by a Wait may avoid clipping the start of the message.

The recording should be present in every codec format allowed for the phone, although .wav (slin) is OK as a substitute for alaw or mulaw. Other format mismatches are possible, but they involve more CPU processing and/or more degradation of the sound quality.

As we are now on the Asterisk, rather than AsteriskNow forums, I have ignored AsteriskNow and assumed you have complete control of extensions.conf, and don’t want to retain compatibility with the GUI.