> even if you spoof the UA from your phone's browser
I don't know about T-Mobile's ways of detection, but AT&T is detecting[0] tethering users by checking the network packets TTL values: If you are tethering then the TTL on their side will be below the expected value of mobile OS's default TTL. There are apps which can hide your tethering usage by altering your device's default TTL. You should still use a mobile browser's UA string, of course.
I don't know about T-Mobile's ways of detection, but AT&T is detecting[0] tethering users by checking the network packets TTL values: If you are tethering then the TTL on their side will be below the expected value of mobile OS's default TTL. There are apps which can hide your tethering usage by altering your device's default TTL. You should still use a mobile browser's UA string, of course.
[0] - http://www.redmondpie.com/bypass-network-carriers-tether-det...