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.
Hello, you have reached the dental office of Dr. X. We are busy assisting a patient now, but will call you back as soon as possible. Please leave your name, reason for calling, and your number. If this is an urgent enquiry, please call 911. In this case, the dentist is giving a reason for not being available and they’re providing a way to prioritize the patients who need the most help. They’re also leaving an emergency number for the most severe of situations.

One cool iOS feature is called Visual Voicemail. This is Apple’s attempt to make voicemail less annoying. Visual Voicemail allows you to see a transcription of the voicemail the caller left. This transcription allows you to read their voicemail as if it were a text message.
If all those steps fail, a reboot may be in order. If you have tried Airplane Mode, deleted the voicemail message, and cleared old messages, this is the next task. Try a simple reboot to see how it goes.