Back in the old days, PLDT owns the only large-scale communications infrastructure: the huge network of telephone lines. Not that it was huge enough: A large part of the country lacked a telephone infrastructure. It was a huge business opportunity but the costs and logistics involved is overwhelming and there were few if any entrants. PLDT enjoyed a monopoly as virtually the only telecommunication provider.
Then there was a disruptive technology: cellular phones. It eventually developed to the point where costs and logistics became more manageable. Quite a few cellular phone service providers (Globe, Smart, PilTel, IslaCom, ExtelCom, DigiTel/Sun Cellular, etc) cropped up to exploit this now lucrative market and compete with PLDT. Of course, PLDT didn’t stand still and soon acquired Smart, the fastest way to get your own cellular phone infrastructure.