The UN has ordered an investigation after Bosnian Serb ex-leader Radovan Karadzic used his limited telephone access to take part in a public event in Montenegro from inside prison.
He gave a speech to people attending the event in Podgorica on Friday.
Karadzic was found guilty of genocide and war crimes by a UN tribunal in The Hague and was given a life sentence.
He was accused of a leading role in the killing of thousands of Muslims in Bosnia during the 1990s Balkan War.
Olufemi Elias, the UN court's registrar confirmed that Karadzic had taken part in the event.
He said that he had now ordered that the detention unit's commander "listens to and summarises" Karadzic's non-privileged phone calls for the past five days.
Karadzic's lawyer said it was "obviously of concern when someone is breaking the rules", but it was what he had said that was important.
Peter Robinson said "it would be better if people focused on his message which actually was one of tolerance" on political issues in the region, including on the role of Nato.
The BBC's Anna Holligan in The Hague says Karadzic remains one of the most divisive figures in the Balkans, and involvement in local debates will raise concern about any lingering desire to wield influence in a still fragile region.
'Systematic pattern of crimes'
Karadzic, a former psychiatrist, was president of the Bosnian Serb entity Republika Srpska during the Bosnian War in the 1990s.
During his trial, the UN tribunal found that Karadzic and other leaders were responsible for the "organised and systematic pattern of crimes committed against Bosnian Muslims and Bosnian Croats".
At Srebrenica, Bosnian Serb soldiers slaughtered nearly 8,000 Muslim men and boys in a "safe area" protected by Dutch peacekeeping forces for the UN.
Judges also held Karadzic responsible for the siege of Sarajevo, a campaign of shelling and sniping which lasted more than three years and led to the deaths of an estimated 10,000 civilians.
After the war, Karadzic hid for years masquerading as an expert in alternative medicine before his eventual arrest in Serbia in 2008.
The 73 year old is currently awaiting transfer to an as-yet unknown state that will offer him a cell for the remainder of his life.