When the mobile telephone was brought to life in 1973, it was big and expensive. The first mobile telephone from Motorola could cost what would be around 1 million dollars.
And because of that it was not accessible to everybody. But today everybody has a mobile phone. Many people, primarily younger people cannot live without their iPhone.
Because the phone is so easy to carry around, it got a new purpose. Then the mobile phone was invented first time, the phone itself were heavy as a brick and could only hold battery for about 20 minutes.
And because of that, you had to consider if it was worth the battery to give this specific person a call, or just wait and talk to him or her in person.