Maricopa county sheriff (MCSO) has some "volunteer" SAR and EMS through their sheriff posse program. But, I think that any medical through MCSO would be secondary to LE and SAR duties. I also hear that MCSO's SAR program is relatively challenging to get into, and that they require extensive initial training. Phoenix FD also has a volunteer community assistance program that uses volunteer EMT's, however they only run behavioral health calls and social work stuff.
@SandpitMedic is right, 911 in the valley is all paid professional, and dominated by Fire. You could look at part time through Rural Metro/AMR, or for a contingent position as an ED tech.