Check the plumber's Arizona licence
Plumbing contractors in Arizona are licensed by the Registrar of Contractors: R-37 for homes, C-37 for commercial work and CR-37 for both. Check a contractor for free on the ROC website or by calling 1-877-692-9762, and make sure the licence is active on the day you sign.
Sources: Arizona ROC: check a license, 21 Oct 2020; Arizona ROC: licence classifications