The cost for the trunks will vary all over the map, literally. Difficult to calculate. If ICW is an instant and overwhelming success . . . Still, ICW does sound as if it could be a viable add-on service for ISPs. First, there's a marketing problem.
Some telcos offer call forward variable, others call forward busy.
Both can produce glitches. If subscribers can only get call forward variable and their computer crashes while they're online, the line remains call forwarded to the ICW server until the customer calls and changes their call forward status. Also not ideal. The ICW server detects that the subscriber is not online and generates a busy signal. With AIN, customers won't need call forwarding.