The main function of an answering machine is to let people leave a message for you when you are not available. In a way the answering machine answers the phone for you. There are combination units that integrate an answering machine and a phone into a single component. The way an answering machine stores messages, determines both the cost andadditional features that your answering machine may have.
Some machines do use two tapes, one for each incoming and outgoing messages.
Since the same tapes are played back and forth they don't last longer. Tapes also have limited storage space. The player reads the exact message from the chip and plays it back immediately. Since there is no rewinding and fastforwarding involved the system does not wear out. Most digital systems have 10-20 minutes of storage capacity.