[gentoo-user] {OT} did my cell carrier block tethering?

[Grant]

Wireless carrier "Carrefour" in Spain seems to have blocked tethering.
 It worked last year but not now.  DNS seems to work but nothing else.
 Tethering does work if I switch SIMs to carrier "Movistar".  I tried
changing my laptop's UA to match my phone's, but no luck there.  I
couldn't get the PdaNet+ app to work but the EasyTether app actually
does allow tethering to work.

Does this sound like it's definitely a case of the carrier blocking
tethering?  Does anyone know how an app like EasyTether works?  Does
all of my traffic route through their servers?

- Grant