One of my first professional works was a telephone provisioning software for cable telephony for a major telecom operator in Portugal, which was used for almost 10 years before the IP wave.
The system interacted with a series of different systems (switches, CMTS, voicemail, CRM systems, all using different interfaces, as you can see in the diagram above) to provision a full service and to provide the support helpdesk with tools to troubleshoot any issues on the customer side.
The system was a based on some redundant servers and some dialog-based applications written in C++ and MFC.