If you’ve already created a custom voicemail on your iPhone, you can tap Play to see how your voicemail sounds. If you’re interested in creating a new message, tap the Custom button. From here, you can start recording your message by tapping Record and tapping Stop whenever you’re finished. Once you’re finished recording, tap on Play to make sure the message comes across as you’d like and hit Save (in the upper right) to save it as your voicemail.
You can also re-record your default voicemail greeting directly from your voicemail menu.

Your iPhone can now send spam calls directly to voicemail — here's how to set it up. But, if your iPhone thinks it knows the number — like if you've called someone who isn't in your phone In one instance, for example, I had this turned on and my colleague's calls kept going right to my voicemail.
For many businesses and professionals, your voicemail greeting is going to be the first point-of-contact for your customers. This is especially true for service businesses, who often rely on their voicemail to collect information from interested parties.