They expect, and should receive, the very best service you have to offer. Most people who are in business appreciate when you keep your word. If it is at all possible try to have a live person answer your telephone in a professional manner. I have called home-based businesses where anyone who happens to be near the phone, including children who are not trained to answer phones, picks up the phone. Also, it is very frustrating to call a company, push 1 for this and 2 for that, then sit through a long message before you can get through to a receptionist.
Human contact is better.
I know that in today's world everyone is busy and voice mail is pretty much the standard, but I personally get tired of talking to machines all day long. Sometimes I might call 10 people in a row and have to leave voice mail messages for each one. When you promise someone you have contracted to work for that you will have something done, have it done. It sounds so simple, yet many times in order to get a contract or keep a contract a person will make a promise they know they can not keep. It is always better to promise long and deliver short.