Unless you can afford to miss telephone calls and the potential business they may represent, you must have some kind of telephone answering service in place at your business. Call waiting, while useful, is not a complete substitute for an answering service because there will be times when you're not around to pick up the phone or when you'll decide it's inappropriate to interrupt a conversation to take another incoming call.
A simple answering machine or a voice mail service provided by your local telephone company will usually do the trick. Because answering machines won't pick up calls when you're on the phone, voice mail may be the way to go if you have only one phone line. In any event, when shopping for a business answering machine or voice mail service, one feature you should probably demand is variable length message recording. This enables your callers, and not the machine, to determine when their messages to you are finished. Another useful feature is remote access, which enables you to listen to your messages from any phone.
Once you have an answering service in place, record an outgoing message that identifies your business and its normal hours of operation and that clearly instructs callers on when they should start their message ("please wait for the beep..."). Keep your message short and to the point to avoid further frustrating a caller who in all likelihood expected to be talking to a live human voice.
Many business owners find that they prefer to have a live human being answer their phones. You don't need to hire a full-time receptionist — you can contract with a telephone answering service that will answer your phones in the way you specify, and then forward the calls on to you. This is especially useful for businesses that want to be able to respond to emergency calls 24 hours a day, or that want to screen calls by some objective criteria.