> was there ever any crossover regarding UNIX folks applying their
developments to other non-UNIX AT&T systems

Besides Sandy Fraser's long-term effort to advance digital communication
(as distinct from digital transmission), there was TPC; see TUHS
https://www.tuhs.org/pipermail/tuhs/2020-April/020802.html and other
mentions of TPC in the TUHS archives.

Ken Thompson did considerable handholding for early adopters of Unix for
applications within the Bell System, notably tracking automatic trouble
reports from switching systems and managing the workflow of craftspeople in
a wire center.

Bob Morris's intimate participation in a submarine signal-processing
project that Bell Labs contracted to produce for the US Navy set him on a
career path that led to becoming chief scientist at NSA's National Computer
Security Center.

Gerard Holtzmann collaborated to instill model-checking in switching and
transmission projects.

Andrew Hume spent much time with AT&T's call records.

Lorinda Cherry single-handedly automated the analysis of call centers'
notes on customer contacts, This enabled detection of significant
human-engineering and public-relations problems.

An important part of my role as a department head was to maintain contacts
with development labs so that R and D were mutually aware of each other's
problems and expertise. This encouraged consulting visits, internships, and
occasionally extended collaboration or specific research projects as
recounted above.

Doug