There are four different types of answering machines currently on the market. The least expensive models use a single tape for both incoming and outgoing messages. The problem with this approach is that callers must wait a long time before recording if the tape is filled with other messages.
More sophisticated machines use two tapes, one for incoming and one for outgoing calls.
This minimizes the delay before recording, since the incoming message tape is kept wound to the appropriate place. Newer answering machines often use digital chips to record incoming or outgoing messages. Models typically combine a digital chip for the outgoing message with tape for incoming messages. Purely digital machines use no tape at all, recording both incoming and outgoing messages digitally. Two-line answering machines, while relatively uncommon, are available for businesses that receive calls over more than one line.