How can I use a locked phone overseas?

Episode 1299 (1:05:18)

Eric from Milwaukee, WI
Apple iPhone 6s and iPhone 6s Plus

Eric has an iPhone 6 Plus and he's heading to London, but AT&T refuses to unlock his phone while he goes. What are his options? Leo says that's because they want him to buy their international data package. Leo recommends renting a MiFi from a local British carrier and use it for his data. He should turn off data roaming on his phone, though. That's how Leo did it and it worked great.