It seems like that would depend on the era we're talking about. Party lines were a thing in the US, too: https://en.wikipedia.org/wiki/Party_line_(telephony) That suggests they were still in widespread use in the 1970s. One imagines that other countries had them for longer yet.

While researching another topic in newspaper archives I recently came across a piece from the late '50s lamenting the number of people in Albuquerque, NM who were still on 4- and even 10-home party lines. By the time of the editorial this was already uncommon in cities (the point of the editorial was to complain that the Bell System had been very slow in upgrading Albuquerque), but at that time it was still very common outside of the few major cities in NM.

Right, it was probably just a symptom for the growth pains of the telecom industry. I would assume that industry had a whole range of extra issues in the Eastern Bloc, beyond the shall we say "natural" challenges of wiring up a whole territory.

In 1984 I recall ordering a phone line in Denver and being asked if I wanted a party line or a single line. My apartment was in Denver-proper and having a party line was still an option in 1984.

We briefly had a 9-line party line at my parent's house in upstate NY in 1989. Their building of the house prompted the local bell to install private lines.

They lived in the country, but not in an usually isolated area. 100% of the areas in rural areas had these in that region until the late 70s.

Some of the infrastructure was just retrofitted to provide private line service. I saw wood based mechanical switches in production (in very isolated areas in New York) circa 1997.