Do I need to pay extra for cellular on an Apple Watch if I already have unlimited data and LTE with a carrier?(T-Mobile)

So I’m planning on buy an Apple Watch Series 3 With GPS and Cellular. I’ve been reading that you need to pay 10 every month to use LTE but I don’t understand what this means. I want to know if my Apple Watch can use my phones/plan of unlimited data and LTE or if I have to get a separate plan for it.