Most mobile networks allow you to contact your voicemail system from another phone. This can be handy if your battery is flat, or if you don’t want to rack up extra costs when you’re overseas.
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.

Can't listen to voicemail iPhone?On your iPhone: Tap Voicemail, then follow the instructions.On another phone: Dial your own mobile number, press * or # (depending on your carrier) to bypass your greeting, then enter your voicemail password.See More...How do I change the voicemail message on my iPhone?
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.