Your prospects are under the impression that a brand will be always available to cater to them, now for some circumstances, a few brands might not be able to serve their prospects 24/7 which is why creating a voicemail message becomes useful.
The NHS *shouldn't* leave voicemail. At least, the bit where I used to work had a policy that one does not leave voicemail, since you don't know who will listen to it and thus you can't ensure confidentiality.[1]

Users often don’t invest enough time into their messages, resulting in incomplete, unprofessional, or otherwise under-whelming greetings. Sure, crafting a greeting doesn’t sound all that complicated; however, there are a number of pitfalls users can fall into—i.e. informality, terseness, sincerity, lack of direction, and more. While none of these sound too catastrophic, they are often interrelated. As such, they tend to worsen any problem. For example, humor can cause informality, worsen ambiguity, and weaken sincerity. That being said, users should strive to avoid ALL these pitfalls.
Hi, we aren’t in at the moment, if you are trying to sell us something please start speaking now and hang up at the beep, everyone else start speaking at the beep and hang up when you’ve finished.