There will undoubtedly be a DC decoupling between the VDSL modem amplifier and the phone line (a 50v DC offset is really not something you want the amplifier circuit to see). I think if you looked at a linecard then you'd find it was probably capacitively coupled to the pre-amp (not to mention the output amp). Of course there's a filter too, but it's really only the voice part of the circuit which has to be filtered. In general most domestic VDSL/ADSL filters just pass the unfiltered signal straight through to the modem. Whether that's how the filters work at the cabinet end on VDSL circuits, I've no idea. There may be more elaborate protection from people doing stupid things like putting mains voltages down phone cables (it will undoubtedly have happened).

Despite what some people seem to think, the filters are there to prevent the VDSL frequencies being effectively "short circuited" by the voice components, not the other way round.

Agreed that the ADSL & VDSL filters are HF-Rejection Circuits, to prevent the xDSL HF signals "disappearing" or at least being attenuated by the PSTN side, such as the phones.

An AF/LF Reject filter would be physically relatively massive, unable to fit in the "dangly things" etc; and would probably still cause quite a fair bit of attenuation to the xDSL signals.

I have taken such a Splitter apart confirming that the filters are in series on the Phone/PSTN side; and that the xDSL side is "straight through".

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Reminiscent electrically to the use of 1/4-Wavelength Stubs in Waveguides (particularly the joints) - although nothing like them in appearance!

[panto]
Oh yes it is!
[/panto]

In reply to a post by Fastman2:

the pstn service is not routed through the dslam

Technically it does pass through the DSLAM but it doesn't require the DSLAM to have power to it to pass through.

However if the DSLAM away damaged say by a traffic accident then PSTN service may be lost also.

Gets as far as the filters on copper strips in the DSLAM cabinet but not through the active electronics of the DSLAM itself which is why pstn connectivity is not lost if the DSLAM cabinet looses power.
If I were a judge I'd have to find Fastman not guilty

If I were a judge I'd rule: close but no cigar.

Fascinating the range of comments on an apparently simple question!

Still, I think we have all learned more - always a good thing.