Edit: I forgot to add this to the title, but I mean in America.
Were they commonplace, or rare/expensive? Would most people know how to operate them? If not around then, what time could they be considered universal?
According to the Scientific American reference book for 1913, there were about 18,179,000 miles of telephone wire in operation in the United States, serving 8,362,000 telephones in 1912.
A New York Times story dated Feb. 1, 1914 describes a debate between Rep. David Lewis of Maryland and former Rep. Samuel Powers of Massachusetts of a measure being considered in Congress. That measure would have nationalized the nation's telephone network in order to prevent monopolistic practices by the Bell company and promote the spread of the network into rural areas.
The story quotes former Rep. Powers, who argued against nationalization:
"There are 12,433,000 telephones in the world, and 8,362,000 of them are in this country -- that is, 67 per cent. There are ten telephones to every 100 of population, one to every ten, and each telephone is used by at least five persons. In the German Empire there is only one and a half telephones to every 100 of population, in Great Britain one and four-tenths to every 100, in France only six-tenths to every 100, and in Sweden three and four-tenths to every 100. In cities in Europe having a total of 62,000,000 of population the average number of telephones in use is two and six-tenths to each 100 of population. In this country, among the same class of cities, there are ten and three-tenths telephones to every 100 of population."
Howard Blum's Dark Invasion tells the story of the German espionage and sabotage campaign against the United States during World War I. The New York Police Department's effort against those saboteurs (who were based in NYC) relied on the telephone network. Police used telephones (including a special dedicated line in their secret HQ) extensively. They wiretapped suspects' telephone lines and identified a key suspect because he used a particular pay phone in a bar.
Even people who didn't have a telephone in their homes were able to use a pay telephone in a store, restaurant or bar with relative ease, if it was within a particular city. Long-distance telephone service was relatively rare, and rural telephone service was also uncommon. The first transcontinental telephone call would not be performed until 1914, and transatlantic calls were more than a decade away. The first one would not be made until 1927.