Forwarding the call after certain ring


I have asterisk with TDM2400 and configured with approximatly 70 extensions. I need to forward the extension to some other number after certain number of rings. One of my user having extension 218, he wants to forward the call to his mobile number after certain rings in IP Phone, so that when he is not the desk, call will be forwarded automatically to mobile number.

Please help me to configure the same as soon as possible.

Thanks and Regards,
Krishna Kumar,
System Admin.

exten => 100,1,Dial(SIP/100,15)
exten => 100,2,Dial(SIP/200)

If you dial extension 100 here after 15 seconds if no one answers Dial will return and asterisk will move on to the next priority.