Not sure what model Inter-tel system you are trying to configure (Axxent, Axxess, 5000, etc) but I think it's pretty universal that you want to setup an Overflow annoucement that will trigger from an Overflow timer.

You can set a call routing application to be the overflow that will give the caller the options you desire.

Again, I'm not sure what system you are on, but this is how it basically works on an Axxess system.