I'd have a lot of fun trying to get around it. For example, if the phone and the computer were on the same non-Internet-connected wifi network, and you set up an SSH server to send outbound requests through the 4G modem, would they be able to find out you're using the hotspot?
There are a ton of methods carriers use to detect hotspot traffic, from the device itself handling the categorization, to TTL values attached to requests, to other very clever network sniffing strategies.
Every method I've encountered in the past was thwarted by a good ole VPN. This was all on unlocked or rooted phones though so YMMV work carrier phones.
I'd just try to disguise the traffic as coming from something else. Someone further down says just switching to an OS that doesn't actively snitch does the trick, but if you really wanted you could make your requests look like just about anything, given added volume is free.