For the first time since 1999, calls can now be made directly from the United States to Cuba and vice versa, without passing through a third country, the company, Etecsa, said in a statement.
"The reestablishment of direct communications between the United States and Cuba contributes to providing better infrastructure and better communications quality between the people of both nations," Etecsa said.
The connection was set up through a February deal signed with New Jersey-based firm IDT Domestic Telecom.
The telephone link between the two countries has been interrupted and restored numerous times since Fidel Castro came to power in the Cuban Revolution in 1959 and began nationalizing American-owned companies in the 1960s.
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But this is the first time the connection has been restored since February 25, 1999, according to Cuban authorities.
Previously phone calls between the United States and Cuba had to pass through a third country, making them expensive and poor in quality.
Around two million Cuban-Americans live in the United States, and many families rely on phone calls to stay in touch across the Florida Straits.
Postal service between the two countries was cut off in the 1960s and has still not been restored.
The new connection will "initially" be used only for international voice calls, but could eventually transmit other kids of communications as well, Etecsa said.
The White House had announced in December that the rapprochement with Havana would include "new efforts to increase Cubans' access to communications and their ability to communicate freely."