There's a fair amount of work on voice over IP and packet switched networks that predates (by a wide margin) 1994, including connections to the PSTN.

For example, see: https://www.ll.mit.edu/news/ieee-milestone-packetspeech.html

And: https://www.researchgate.net/profile/Stephen_Casner/publicat...

And: https://qos.ittc.ku.edu/research/thesis/documents/michael_to...

etc.