The International Committee of the Red Cross (ICRC) says it lost direct contact two weeks ago with three of its aid workers being held by al-Qaida-linked militants.
But the Swiss-run aid agency says it heard one hostage in the Philippines speaking on the phone on Friday with a television reporter.
At the end of January a Philippine government official visited the three ICRC workers at a jungle hideout near the Jolo Island's Indanan township.
The ICRC believes Swiss national Andreas Notter, Filipino Mary Jean Lacaba and Italian Eugenio Vagni are in stable health after 50 days of captivity and it still hopes to secure their release.
The leader of the Abu Sayyaf militants has said he will not negotiate unless the Philippine military pull out of Jolo island jungles. The military has rejected these demands.
Abu Sayyaf has about 400 members and is on a US list of terrorist organisations for its links to al-Qaida and involvement in kidnappings, bombings and beheadings.
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