Are prank calls legal?

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I don't even know if it counts as prank calling... but basically someone has been calling my house line and mobile, then cutting off. Its not a problem if its once, but one night it was around 8 times at past 1am in the morning. just had a couple now as well. Basically just wanted some opinions.. how much grief/hassle would it be to formally complain to authorities and is it even illegal?? Luckily they were stupid enough to not withhold the number the first time round, but now they are with holding it. __________________