No Ringback Tone

I just installed asterisk version 1.2.4 and I can call a PSTN number (via a Cisco Gateway) using SIP Phone. I can hear the PSTN phone ringing (as it is also on my desk) but I don’t hear ringback tone on the SIP Phone.

Can anyone please help me?

On extensions.conf, I have these lines:

exten => s,1,AGI(astlocal.agi,${ARG1})
exten => s,2,Dial(${DIALSTRING},20,r) ; Ring the interface, 20 seconds maximum