BEIJING — The hotline rings, but nobody answers.

China’s Ministry of Public Security opened the line last month to answer questions about the new law regulating foreign nonprofit organizations, which takes effect on Sunday.

But this week and last, calls went unanswered, exemplifying the uncertainty that still surrounds the law, raising concern among thousands of nongovernmental organizations about their ability to continue their work in the new year.

The law, which places a raft of new requirements on foreign nonprofits operating in China, is another building block in President Xi Jinping’s fortification of one-party rule, which he sees as threatened by foreign influence and unfettered civil society.

Under the law, foreign nonprofits such as foundations, charities and many business associations must register with the police, persuade state agencies and organizations to act as their sponsors, and submit regular, detailed reports on their activities.