Back in the early days of the Internet, I met some of the Bell Labs people working in that area. What they didn't like about an IP-based network was the jitter. That was totally unacceptable for voice. They wanted something with reliable clocking, and had come up with Datakit. That sends all packets for a given call over the same path, in order. You don't open a virtual circuit unless all the nodes have enough bandwidth for it. Telcos still use Datakit, and Asynchronous Transfer Mode is a successor to it.

I never dreamed that people would accept the degradation of telephony to 1 second of delay jitter with random dropouts and echoes.