In general if the phone does not have a very obvious tethering option the ISP want to change extra for you to be able to do that. Most times this is tied to the hotspot options too since they disable those also unless you pay extra.
Many people root the phone to bypass this restriction but if do you generally need to also run a VPN since it is really obvious that you are running tethered if they see flash based video running that cannot run on a phone.