When you're alone on a call and feel like a fraud

I finished my apprenticeship a few months ago, and today I stood alone in a stranger's basement staring at a unit I couldn't figure out. The customer was waiting, my chest was tight, and all I could think was that any second they'd realize I don't know what I'm doing. I've got the papers that say I'm qualified, but out there on my own, I feel like a fraud who's about to get found out.