Comment by skissane

The software package they were using was an Ericsson product, called "JZA 715", for PDP-11s. Did Ericsson ever give them the source code? Possibly not.

Does Ericsson still have the source code for a PDP-11 software package they sold in the 1970s? Maybe they do, maybe they don't.

Did they ever even ask Ericsson for the source? Possibly, they may have thought that modifying PDP-11 software was too hard–people with skills in doing it are hard to find these days, it is unlikely to be written in C, probably either assembly or some obscure language few have heard of–so even if Ericsson still had the source and was happy to hand it over (free of charge or for a reasonable fee), they might not have thought it worth their while to acquire it, and hence never have asked Ericsson for it.

EDIT: Actually, turns out JZA715 is mostly written in Pascal (so not that obscure a language after all) with some modules in PDP-11 assembly – see page 18 of http://srsv.org.au/wp-content/uploads/2017/02/S-38-1-Jan.pdf – the software ran under RSX-11M