[throwaway81523]

I don't think latency and jitter are big issues. If you're imagining VOIP packets flowing through the Erlang app, it's more likely that they are routed by hardware at a lower level. The Erlang app controls the lower level switches. I don't know about Ericsson switches in particular, but I've worked on telecom stuff (not programmed in Erlang, unfortunately) and that's how it worked. The low level routing was done by FPGA's controlled by a C program. The C program could have been written in Erlang instead, and that would have made life a lot nicer for the programmers.

[rcarmo]

You'd be surprised exactly how much functionality is written in vanilla C and pushed down the stack to raw packet handling. There is hardware acceleration in NICs, but voice is actually a tiny, almost insignificant amount of the workloads involved in some parts of the world.

Think hyperscaler-style SDN, but largely on-premises.