WASHINGTON — The National Security Agency’s once-secret program that is collecting bulk records of Americans’ domestic phone calls is taking in a relatively small portion of the total volume of such calls each day, officials familiar with the program said on Friday.

While the agency is collecting a large amount of landline phone data, it has struggled to take in cellphone data, which has undergone explosive growth in recent years and presents additional technological hurdles, the officials said.

The revelation came days after the nation’s secretive Foreign Intelligence Surveillance Court approved President Obama’s proposal to impose new restrictions on when and how analysts with the N.S.A. may gain access to the raw database containing the bulk phone records, according to the Office of the Director of National Intelligence.

The bulk call records program began under the Bush administration and was based on claimed wartime powers. In 2006, the program was brought under the surveillance court’s authority. It came to light after leaks by the former N.S.A. contractor Edward J. Snowden.