@Curtis was more interested in attempting to get removed from mass lists by having the not-in-service tones play, but your way is probably the way to go if you care deeply about possibly legitimate callers calling from toll-free services.

This got so bad for us - double-digit spam calls an hour - that I essentially blacklisted all toll-free numbers. The blacklist module in FreePBX wouldn't do this correctly - I can't remember why - so I

1. created new inbound routes
2. set DID for Any and create a route for:
   _800NXXXXXX
   _822NXXXXXX
   _833NXXXXXX
   _844NXXXXXX
   _855NXXXXXX
   _866NXXXXXX
   _877NXXXXXX
   _888NXXXXXX
3. set destination to Terminate Call: play no service message

Immediately cut down by 95% the spam calls we were getting. I am sure we lose some legitimate callers but the problem was getting so bad the collateral damage is acceptable for the payoff.