Sounds similar to here in the US, where Bell Systems had a monopoly on the telephone service market. Prior to 1984, any telephones or accessories had to be rented from Bell Systems, and wiring had to be installed by them.
There were ways for the telephone company to know if unauthorized additional telephones had been connected to the line within a home. Many telephones were wired directly into the wiring, although you do still see some old telephone having the four prong style telephone jacks. The modern modular telephone jacks replaced them during the 1970s.
In 1984, everything changed, the Bell System was split up, and at this point customers were free to do their own wiring and purchase their own telephones as well as wall jacks and accessories at AT&T retailers. The modular jacks of course, were a popular part of that.
Prior to 1984, unauthorized imported telephones and jacks, etc were imported via other countries such as Japan. But these were technically illegal by phone company standards.