The phone line is meant to serve as a tool to defuse potential flash points along the heavily militarized border between the two Koreas.
But Andre Kok, deputy public affairs officer for U.S. Forces in Korea, said that reports of the North's Korean People's Army, known as the KPA, cutting off communication often arise when military training exercises are taking place, as they are at the moment.
"When we place a call on the direct phone line and the KPA does not answer, we have no way of knowing if the KPA has actually disconnected the phone lines or are just not answering the phone," he said.
On Saturday, North Korea's foreign ministry said sanctions against the North over the last eight years have only caused it to bolster its nuclear deterrent program "qualitatively and quantitatively," according to the North's official news agency, KCNA.
On Friday afternoon, Pentagon spokesman George Little said North Korea "will achieve nothing through threats of provocations, which will only further isolate North Korea and undermine international efforts to ensure peace and stability in Northeast Asia."
Little said the United Sates "calls on (North Korea) to refrain from further threats and provocative actions," and said the United States is fully capable of defending itself and its allies, including South Korea and Japan.
New U.N. measures
Tensions are particularly high at the moment because of the new measures against the North adopted unanimously on Thursday by the U.N. Security Council.
"These sanctions will bite, and bite hard," U.S. Ambassador to the United Nations Susan Rice said after the vote.
China, North Korea's key ally, could have used its veto power to block the sanctions. Instead, after weeks of negotiating, it signed on to the final draft.
"China is a country of principle," China's U.N. Ambassador Li Baodong said. "We are firmly committed to safeguarding peace and stability on the Korean peninsula."
On Friday, China's Foreign Ministry called upon "all relative parties to stay calm and refrain from taking actions that may escalate tension." Spokeswoman Hua Chunying reiterated China's support for the sanctions, which have stoked Pyongyang's fury.
The goal of the new sanctions is to stymie the activities of North Korean banks and cash couriers who might be funneling money to the secretive regime's nuclear and missile programs.
It will be tougher for the regime to move large sums of cash stuffed into suitcases, Rice said.
The U.N. resolution also outlines measures to step up scrutiny of suspicious sea shipments and air cargo and it expands restrictions to encompass several institutions and senior officials in the North's weapons industry, as well as a range of materials and technology known to be used in uranium enrichment.
It also blocks the sale of luxury goods, such as yachts and certain high-end jewelry, to North Korea.
"As a result, North Korea's ruling elite, who have been living large while impoverishing their people, will pay a price" for the ongoing nuclear activities, Rice said.
Questions over sanctions' effectiveness
Some doubt whether the new measures will make much difference.
Sanctions imposed after previous nuclear tests and rocket launches have failed to deter Pyongyang.