If you want them to leave a voicemail, tell them right from the beginning, “Please leave your name, reason your calling, and the best way to contact you.” If you want them to call you back during normal business hours, tell them, “Please call back during normal business hours.” If you want them to call back prepared for a lengthy conversation, you can say something like, “Please refer to our website’s FAQ page,” and then provide the URL. Self-service should never be underestimated.
Tips and 10 scripts for professional voicemail greetings. Discover the best practices and ready-to-use examples for recording a business phone greeting by a professional voice artist. Often your voicemail message is the first point of contact with potential business clients so it should be worth listening to.

Also known as analog lines, POTS is the traditional method to support making and receiving phone calls over the Public Switched Telephone network. With the many benefits of data network services, there aren't many cases where POTS lines are the best way to go. Usually they are only need when a device needs a loop start. Gradually, the ILECs are phasing out POTS service and in many areas of the country, you can't even order it, anymore. For instance, Hurricane Sandy wiped out most of the copper infrastructure in New York/New Jersey and Verizon did not repair. They replaced it with fiber and wireless infrastructure.
Your message should be 20 seconds at most. Remember, your clients or co-workers may listen to this dozens of times.