What it used to be called:
Cell phones became indispensable.
The term itself goes back a long way. The original landline referred to a wire carrying telegraph signals over land rather than under water. Alexander Graham Bell's telephone system used land line for a wire carrying voice signals.
However, the word wasn't used much until cell phones entered the conversation. At that point, landline came to describe the immobile, newly old-fashioned telephone.
An early use (of the modern sense):
"When, if ever, will the migration from landlines to mobile stop, and when will landline networks cease to be economically viable?" McKinsey Quarterly, Jan. 2000