The Bell Labs people had their own approach - Datakit.[1] This was a circuit-switched network as seen by the user but a packet switch inside the central-office switches. Bell Labs used it internally, and it was deployed in the 1980s.
Pure IP on the backbone was highly controversial at the time. The only reason pure IP works is cheap backbone bandwidth. We still don't have a good solution to datagram congestion in the middle of the network. Cheap backbone bandwidth didn't appear until the 1990s, as fiber brought long-distance data transfer costs way, way down.
There was a real question in the 1980s and 1990s over whether IP would scale.
A circuit-switched network, with phone numbers and phone bills, was something AT&T understood. Hence Datakit.
Pure IP on the backbone was highly controversial at the time. The only reason pure IP works is cheap backbone bandwidth. We still don't have a good solution to datagram congestion in the middle of the network. Cheap backbone bandwidth didn't appear until the 1990s, as fiber brought long-distance data transfer costs way, way down. There was a real question in the 1980s and 1990s over whether IP would scale. A circuit-switched network, with phone numbers and phone bills, was something AT&T understood. Hence Datakit.
[1] https://dl.acm.org/doi/pdf/10.1145/1013879.802670