When call centers first came about in the 1960s, they were built over PBX systems. Private Branch Exchanges (PBX) are essentially a small-scale version of the telephone exchange and are typically handled by one of the few telephone network providers in the country. Fast forward to today, a call center software provider does not need to own even the VoIP line that is necessary to handle communication. The software can simply connect to third party VoIP services with the help of an API and get started on their service.
The cloud has brought down the cost of operating a business and while this is a good thing for the consumers, it also means a rise in the number of competitors one has to deal with.