Is it possible in PbxAct / FreePbx when a voicemail is left in a mailbox it can send a text back the caller and say thank you for calling, we will return your call as soon as possible.
(of course, it will only go through if they have a cell phone)
Not legally, at least not as a default. You have to receive active consent to sent an application based SMS. So if you go through the process of getting consent then you could automate it for numbers you have received consent for.
Yes, I completely forgot about consent, so the Voicemail will need to say something like if you would like to be notified by text Press 1 or have some kind of opt-in