Yes, the answer is to send the call somewhere that costs nothing. There's lots of options depending on how your sp1 and sp2 are set up.
For example, if you have a voip service provider set up on sp1 that allows free 0800 calls (such as sipgate), then this is a good one:
Physical Interfaces->Line Port->InboundCallRoute: {(0|00[0-79]x.|01612448121|02070609904|08009777600|08446882753|02070605576):sp1(0800500005)},{ph}
It's British Telecom news about itself – boring and goes on forever
As the calling party is not speaking, the call will terminate after one minute at default if you have DetectFarEndLongSilence checked in the Line Port settings. Uncheck it if you want to keep the call going for as long as the calling party hangs on.
There are lots of fun places that you can send the calls on the web for free. Post back if you wish to know more.