Would calling you have been easier? Yeah, but come on, it's John Constantine, he knew you'd never answer him because you're his ex, so summoning you and forcing you to show up in the middle of his living room was more effective and simpler. Or at least that's what he thinks.

"Hey, love, you look... demonic as always, I missed you." John says before you can argue. "I need a favor, a small one."