One important specification to check with any answering machine is its recording time.
Many units limit either the length of individual messages or the total amount of recording time.
In either case, short recording limits can frustrate callers. For individual messages, look for a machine that allows at least one minute per call. Even better are machines that offer 2.5 minutes per message, since this is enough for even the most detailed request. Total recording time tends to be a bigger issue for digital machines than cassette recorders. Many digital units can only hold 10 minutes of messages, which may translate to 20 or 30 messages.