Although the feature is often called "Teletext passthrough", it is actually far from that simple - the STBs need to explicitly do stuff to have the Teletext service on the signal that comes out from them. Teletext is delivered in a completely different way over a DVB transmission that it was in the old analogue times.
Some STBs don't have a passthrough feature at all, so you have to use the Teletext function of the box itself. Then there are some that only have the passthrough, and then there are some that have both.
I have to admit I have no idea how this works if the box and the TV are connected digitally over HDMI, but it will still definitely need support from the STB.
Also, where are you located, OP? For example in the UK the digital Teletext service is implemented in a completely different standard (MHEG-5). I don't suppose that will ever work through an STB, you need to use the STB itself to view it.