I had the same problem and the only way I was able to accomplish it was by using a voicemail blasting instead of the user specific mailbox. Either if you use a system recording or the default “Read group number” as Audio Label, the system will by default ask you for a confirmation (“If this is correct, press 1”). If there is no action from the caller, the system will automatically disconnect the call after a 20 seconds timeout.
I hope this helps!