ChildLine is not a type of telephone. It is a name: a British group with a free phone line that children and young people can call if they are being hurt, or if they have a problem they cannot tell their family. The line here is a phone line. You write it ChildLine, with two capital letters, because it is a name, like a company. You will meet it in British news, school materials, and TV. It is a UK thing; other countries have other names for the same idea. Though actually, the idea itself is simple everywhere: a number a child can ring to get help from someone who is not mum or dad.