"Dear Mr General-Secretary Ban Ki-moon, please save us... We don't want to die."
The authenticity of the call could not be verified. It came as Afghan doctors air dropped medicine intended for the hostages, the head of a private Afghan clinic said.
The medicine was dropped in an area of desert in the Qarabagh district in Afghanistan's Ghazni province on Sunday, as instructed by the Taliban.
Mohammad Hashim Wahaj, the head of the clinic, said that his team had dropped more than $1,200 worth of antibiotics, painkillers, vitamins and heart pills.
"It was a big risk, but we had to take the risk because it is a humanitarian issue," he said.
Two hostages have been shot dead since the group was seized and the Taliban has said that more captives will be killed unless some of their fighters are released from jail, a demand the government has rejected.
The remaining hostages are said to be ill.
Shadow over summit
The hostage issue is likely to cast a shadow over two days of security talks between Hamid Karzai, the Afghan president, and George Bush, his US counterpart, which began on Sunday.
The presidents are also expected to discuss the increasing number of civilian casualties sustained during military operations in Afghanistan, as well as the country's booming poppy trade.
A South Korean delegation travelled to Ghazni last week seeking face-to-face meetings with the kidnappers to try to break the deadlock, but there has been no agreement on where to hold direct talks.
The South Koreans, a group of church volunteers who were undertaking aid work in Afghanistan, were snatched from a bus in Ghazni province on July 20.