You are denying the network revenue by using them to send a signal free of charge. They will tolerate people using things like toll savers on answering machines, because they still expect a lot of chargeable traffic, but if you have a business that never generates chargeable traffic, they are likely to consider that it breaks their fair usage rules.
A SIP trunk is an example of a trunk where you can send alerting (180 Ringing) and then reject the call (e.g. 4xx Congestion). A simple, POTS, analogue line is an example of one that doesn’t support this. I think ISDN lines are closer to the SIP case.
SIP doesn’t provide a means to reject a call with ringback and I doubt that ISDN signalling does, either. The way the call finally gets rejected is likely to depend on the originating network and all the intermediate gateways. However, I would expect it to result in an NU, BUSY or CONGESTION tone, or a voice annoucement.