It appears as though since Katy called Lily "fat" Lily wants to respond much like LiLo would by using the internet and posting Katy's cell number on her Facebook page.
According to The Sun,
Writing on her Facebook page, Lily said: “I have Katy Perry’s number, someone did me a favour.
“I’m just waiting for her to open her mouth one more time then it hits Facebook.”
Cell number? Come on girls can't we all just kiss and make up?