occurs when somebody makes a telephone
call and hangs up after one ring.
The receiver of the call is not expected to answer, but he or she will have an idea what the phone call is supposed to be about, usually predetermined in a previous communication (they will know who made the squillo if they have caller ID functionality on their own phone).
Otherwise there are some standard squillo meanings depending on the context: if you are waiting for somebody and you receive a squillo from that person, then they are saying they are running ten minutes late (or thereabouts). Alternatively if you are rushing to an appointment and you receive a squillo from the person you are going to meet, then they are wondering where the hell you are (mind you, it could also mean the other person is late too). If you are having a conversation with somebody and that person hanged up suddenly, and you shortly receive from them a squillo, then that person has run out of credit on their mobile phone, and would you be so kind enough to ring back. And if you are a young girl and you keep getting squillos from a complete stranger, then that person is showing interested in you. Express your mutual interest by responding with...a squillo.
The word originated in Italy as a handy method for cash strapped students to send messages, as neither the caller nor sender pay anything. The word comes from the Italian word 'to squeal' (squillare).