United WiFi TOS explicitly forbid the use of VoIP.
From
https://www.united.com/web/en-US/con...wifi/faqs.aspx
Although you are allowed to use your cell phone in airplane mode for activities such as surfing the Internet and sending email,
we do not allow the use of our onboard Internet service for voice communication or videoconferencing through a cell phone, VoIP or any similar service. Voice and video calls are not permitted. By purchasing an Internet plan and accessing our service on board, you agree not to make voice or video calls, including but not limited to VoIP, in flight.
edit: 474SP beat me to it.
Let the crew handle this. Get up, flag a FA and ask them to remind the offending passenger that using facetime in flight isn't allowed.