The general voicemail greeting is used as the default voicemail greeting if you don't set any other greeting type. Occasionally, during events like power outages, even if you've set another greeting, your personalized programming might be wiped out. Or you might simply forget to change the greeting after returning from vacation.
If they are calling without an expectation that you always pick up (such as if you are a business coach or a one-man shop) then thanking them for calling might make the most sense.

4. Enter your voicemail password. If you have set a password lock for your voicemail account, you will be prompted to key it in. Use the dial pad to do so.
I have a confession to make: I haven’t recorded a new voicemail greeting since 2014. In the past four years, I (hopefully) have become more articulate, poised, and self-assured. But hear my voicemail recording, and you’d think I was still new to the work world, a little unsure of myself — and probably not an authority. Obviously I need toupdate it. And if you haven’t changed your voicemail greeting in over a year, you’re likely in the same boat. After all, a professional voicemail recording boosts your credibility, makes you seem more competent, and encourages whoever’s listening to it to continue the relationship. A relatively unprofessional one — like mine, for instance — does the opposite: It encourages prospects, recruiters, and potential connections to run in the other direction. With that in mind, I’ve written 18 unique voicemail greetings for every situation. Pick your favorite, practice a few times, then record your new voicemail.