> Mobile carriers do not use VoIP with the MS

No, they do, exclusively. LTE and beyond don’t even support circuit switched calls anymore.

Bellhead wasn’t intended in a derogatory way, just as a reference to the “Netheads vs. Bellheads” schools of thinking about networks.

I do have great respect for historical phone systems and the clever engineers making them work. In terms of absolute reliability, I think VoIP was indeed a step back (although I think that’s mainly due to modern engineering and QA practices than inherent limitations).