The NOOP dialplan and, in fact, no dialplan at all, will hangup immediately, without answering the call.
Note that I did suggest you look into this option, as it might be that the service provider assumes an early media indication is a chargeable event:
; PRI Out of band indications.
; Enable this to report Busy and Congestion on a PRI using out-of-band
; notification. Inband indication, as used by Asterisk doesn't seem to work
; with all telcos.
; outofband: Signal Busy/Congestion out of band with RELEASE/DISCONNECT
; inband: Signal Busy/Congestion using in-band tones (default)
; priindication cannot be changed on a reload.
Maybe your service provider is aware of what you are doing and has added some special rules for you, as I almost commented in the first place that this might be considered a fair usage breach. You are getting a service without covering the cost of that service.