They were using dial up ISDN lines to send the data back, but Riposte didn't support that, or scale to 20k terminals, so that was all new code
In general they had a distributed database that couldn't do ACID
https://www.postofficetrial.com/2019/12/fisking-horizon-tria... https://www.computerweekly.com/news/252496560/Fujitsu-bosses... https://www.benthamsgaze.org/2021/07/15/what-went-wrong-with...