Protip, you can delete the FaceTime application on an iPhone.

decline

"I'm not busy but I'm not currently accepting calls. What's up?"

Although I usually respond to čare you doing something/busy/things like that with "why, what do you need?" since my family loves trying to guilt trip people who have just said they aren't doing something into doing whatever bullshit they're calling for...

"I don't want to be busy either."

decline

“sorry I don’t have my headphones with me and my phone’s mic has been acting up. What’s up?”