Session establishment for PSTN can be relatively expensive (much in the way of TLS handshakes), so concurrency and shared-nothing memory model together allowed for real-time streaming to keep on working no matter what else happened. The three main features of a PSTN switch are, after all:

1. Reliable call switching

2. Reliable real-time throughput

3. Reliable billing and accounting data generation

We don't think much of throughput these days, when any home office switch has gigabit ports and 40Gb+ backplane. As far as I know, maximising throughput bandwidth was not a primary consideration with Erlang. Reliable real-time streaming is much more about guaranteed latency - and incidentally, optimising between latency and throughput tends to be all about tradeoffs.