United Nations peacekeeping staff in Eritrea have rung up more than $500,000 of unpaid international calls.
UN staff stole and distributed pin codes to make 'free' calls
The fraud was discovered last year when auditors noticed huge billing discrepancies in 2003, the UN said.
Schemes such as stealing pin codes and abusing a one-minute grace period before being charged for a connection accounted for the "irregularities".
The countries of those caught swindling their phone bills have been charged, but so far only $14,000 has been paid.
The UN Mission to Ethiopia and Eritrea (Unmee) said the process of unravelling the fraud was "painstaking and complex, involving the manual verification of 1.4m lines of computer billing data".
UN staff are affiliated to peacekeeping missions from their country's team at the UN headquarters in New York.
To avoid absorbing the cost itself, Unmee has forwarded $364,000 of confirmed bills to New York.
Since 2000, a 3,000-strong Unmee peacekeeping force has patrolled Eritrea's tense border with Ethiopia.
The two Horn of Africa countries fought a war between 1998 and 2000 that is thought to have killed more than 70,000 people.