Iremember doing exactly this kind of hack for a small telco in Bueno Aires. Extel. Around the year 2000.
In most cases it was not much more difficult than what OP described.
I worked on some software that was used by telcos around that time - you were probably hacking our dongles :)
I worked on some software that was used by telcos around that time - you were probably hacking our dongles :)