Telephones are nothing new. They have been around since the turn of the 20th century, providing us with connectivity and convenience. Today, an average person can have a whole slew of phones and a different number for each. Maybe you have a mobile phone, home phone, and a work phone.
This can lead to problems. You have to keep a bunch of numbers committed to memory and give an acquaintance the appropriate number. If they are a co-worker, probably your work number, a friend, your mobile, and so on.