Is it legal for an insurance company to raise rates if you are simply calling for a quote to determine the cost of another driver on your policy but have not officially placed that person on it yet?

Maybe the agent saw something that had been overlooked and that caused your rates to go up. Check with a different company. Shop around for rates and see what you can get. If you think the agent is messing with you, don't put up with it. But the simple fact is, insurance companies can charge whatever they want and change the rates whenever they want. Your only protection is to shop around. Competition keeps everybody honest. Insurance companies can run a check of licensed drivers by address. If they find licensed drivers in a household that are not listed on the policy they may add the additional drivers that they found. I would have to ask though, are you sure the change in premium was related to your call? As was said in the first answer, shop around if you do not trust your current agent.