Zero for the operator
Make sure that callers in the voice mail system can reach a human at any time. In general, it is a good idea to use the zero key to reach a live operator.

Voice mailbox rentals
One alternative to buying a voice mail system is to rent mailboxes from a service bureau. This can be particularly helpful for companies that do not have phone systems that work with voice mail. Service bureaus typically charge each subscriber on a monthly basis, with some adding charges for connection time, saved messages, or the number of messages you receive.

Maintenance
A voice mail system will require occasional reprogramming, as well as (hopefully infrequent) emergency repairs. Your maintenance costs should typically not exceed 5%-10% of the purchase price per year.