My son did take an ambulance ride last year, so it's probably a legit company. I'm sure people would remember if they had this service provided to anyone on their insurance plan or a minor they are financially responsible for the debts of.
Caller ID says only "Phoenix, AZ" so this is a tip off it's not a valid company. A woman's recorded voice said that the call had to do with a "recent transition you had with us" but didn't name the company. They asked for a call back regarding this supposed transition. I don't think so!