[cisco-voip] Ideas on how to accomplish this ?

[Tech Guy]

OK so I got this user request from a small dept. within the company. 
They are looking at setting up a toll free number for a survey.  They
want the number to come in and ring an extension for 3 rings, then to
another if no one answers, and then to another.  Finally if no one
picks up it should go to voice mail.

I have done stuff on CallManager for individual lines where you set
the forward no answer or forward busy to ring to another extension. 
But if I do that on this then all of their calls would be forwarding
in this manner, and that would not be good.

So I am thinking I am going to have to get this 800 number mapped to a
new internal extension so that I am not making changes to the
individuals extensions.

I am a little confused about the best way to accomplish this to be
honest, and so I thought I would ask for some ideas from others.

Thanks, hope this is not too far off topic.