From Criminal Records, an article from the Sunday Times Magazine on UK pirate radio:
There is a studio mobile too. It vibrates every few seconds like a faulty alarm clock, as listeners call and text. Scrolling through its inbox, I notice scores of "missed calls". Big N explains that this is how pirates gauge a record's popularity. If listeners like a tune, they call in and then ring off, so the studio mobile registers a "missed call". This costs callers nothing. If Xtreme receives over 20 missed calls from different numbers before a track ends, the DJs play it again. This is why teenagers listen to pirate radio: it's interactive in ways legal stations can't match. Some tune in on their mobiles - on the bus, in the high street, even at school.