Please realise this is a very poor workaround for your carrier's ineptitude, and should be avoided if at all possible. Rearranging the code to make this setting available on a per-rate basis is not impossible, but you'd need to be familiar with PHP & SQL. I doubt it's worth the effort. If a carrier can't even get something as obvious as answer supervision working correctly it indicates they don't have a clue about telecoms and VoIP.
Find a better carrier who doesn't defraud you on each call you make. If you don't vote with your wallet these shoddy carriers will proliferate, and then you'll be left with no option other than to pay these jokers to listen to their fake alerting whilst the outbound leg has probably already failed.
I thought we'd left the old, dark days of dodgy VoIP termination behind us years ago.