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.
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