At one time, companies had separate infrastructures for their telephony and their computer network. You made phone calls via a cable to a telephone switchboard, used the internet via a separate network cable that was connected to your computer network. Every employee needed his or her own cables. If your company had multiple offices, you had to install a separate telephone switchboard for each physical office. The situation was intolerable for businesses that were growing steadily or had multiple offices.
The first optimisation came about when this combination of analogue and digital switchboards was replaced by a single, overarching main switchboard. In that setup, the phones were connected to the switchboard via VoIP, i.e. via the computer network. Access to your operator and the outside world was still based on the old ISDN technology. That solution was pretty expensive and not entirely scalable. Depending on how many employees you had, you needed several ISDN lines, generally with the inevitable extra hardware on your switchboard. There were many instances where the switchboard couldn't take this hardware, so you had to buy a new one.
In past years, ISDN has been replaced more and more by VoIP. No more ISDN line to your operator, with VoIP you use your internet connection to connect to the public telephone network as well. More than that, you can opt for a switchboard in the cloud and all the benefits that this brings.