And using a calling card from a payphone can be significant these days since the payphone owner can now "legally" extort huge charges from the long distance carrier or card provider, who will then extort those surcharges from us.
Answer supervision is indeed passed back from cellular service switches. But remember that these days (and even in times past as well), standardization is not perfect. And it isn't always consistant as to which cellular providers will supervise back your calls to such messages. It can vary from switch to switch within the same cellular provider as to whether you are charged or not for reaching such a "subscriber not available" (vacant) message.
Even with traditional landline providers, sometimes you can find a charge condition on reaching intercepts (which are really the digit by digit quote back systems), and "vacant condition" recordings. And as I said, there were times this happened even in years past, back when the telephone industry in the US was still mostly managed by AT&T and Bell.
If you are concerned, make a note of such calls, and check to see if they were indeed billed when the bill arrives. And then complain to your long distance company or card provider to get a credit.