A pensioner in Rome raised the alarm after receiving a call, apparently made at random, from migrants on a sinking boat in the Mediterranean, reports say.
The person on the boat reportedly dialled numbers with a Rome prefix and eventually got through to a 67-year-old man in the Marconi district.
The man said the caller had spoken some English and French and the sea could be heard lapping in the background.
Police alerted the coastguard who then launched a rescue mission.
It came as the Italian navy and coastguard said their vessels had rescued more than 1,300 migrants in several operations on Tuesday in the Mediterranean.
So far this year, more than 11,000 migrants have arrived in Italy by sea and about 100 have died making the crossing from the North African coast.
The EU is seeking to limit the numbers reaching Greece via Turkey.
The pensioner told Italian media he had been woken early on Tuesday by the telephone and, despite not being able to understand the caller, had contacted the police as he believed the person was in trouble.
A police officer was at hand when the person called back. The caller, who said he was Sudanese, shouted “we are in trouble” amid the sound of an engine and the wind and the sea in the background, Italy’s AGI news agency reports.
Italian newspaper Corriere della Sera says the coastguard later rescued about 600 migrants on a boat and four rubber dinghies close to the coast of Italy.
The number of people risking the perilous crossing from North Africa to Europe is expected to increase as the weather improves, correspondents say.
Hundreds of thousands of refugees and migrants have arrived in Italy in recent years, seeking a better life in the West.
The EU and Turkey recently struck an agreement aimed at halting the influx of “irregular migrants” through Greece.
About 144,000 irregular migrants have arrived in Greece this year alone, and more than 360 have died, according to the International Organization for Migration.
Last year, more than one million migrants and refugees, many fleeing war in Syria and Iraq, reached Europe by sea.
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries such as Syria, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.