AT&T plans to boost its number of fiber locations to 45 million by the end of 2029 but says it isn't profitable enough to build fiber to the other parts of its old landline phone and DSL networks.

AT&T agreed to buy Lumen Technologies' Mass Markets fiber internet connectivity business for $5.75 billion in cash, sending shares of the Denver-based firm soaring in premarket trading Thursday.

Dallas-based AT&T announced the tentative $5.75 million deal to acquire a majority of Lumen's fiber internet business. The deal still needs regulatory approval. Skip Navigation ...