Tax on internet sales?

When I was buying a part online I started to check out and realized I had a question on my order. The total had tax calculated in with free shipping. I called the company to get my question answered and decided to pay over the phone. The lady gave me the total without tax so I question it why no tax. She proceeded to tell me if I buy on the internet I will have to pay internet tax. If I buy over the phone there’s no tax.