At least 300 million people are using phone created by somebody to chat online.
为什么句子中可以省略who are
At least 300 million people are using phone（which are) created by somebody to chat online.
所有的现在分词或过去分词在跟在一个名词后修饰该名词的时候,这其实都是一个省略了which is 的定语从句,都可以省略（which are)