Until recently all telephones were analog. That is, the audio signal is completely processed via analog technology rather than digital. Now, some phones are "digital" and they use a digital signal (like a computer) that represents the sound.
Digital phones are frequently used in offices where the digital functions support voicemail and many other computer-like operations in addition to the actual processing of the audio signal. These digital phones used in offices still receive an analog signal from the phone line, but process it and handle the other new functions digitally.
Although these digital phone systems are new and have a lot of neat features, they also present problems for people with hearing loss. For example, you cannot use certain patch cords or FM systems which plug into the phone jack of an analog phone to link your hearing aid to the phone.
Another type of phone which is digital is the digital cell phone. It also presents certain problems for people with a hearing loss.