Is there a reason why some trains act like faraday cages regarding mobile signals? It doesn't just affect Voyagers I've had similar problems on TPEs 185s, when the doors are open its fine but as soon as the doors close the signal goes again.
It's more to do with the windows
1) They are often smaller on modern trains
2) They often are not next to where hold phone by ear, instead there is a pillar (and some have wide pillars)
3) Glass is often coated (or tinted) to reduce glare and effect of sun warming inside (to assist air conditioning).
4) Films bonded to glass can also be a problem (often added to avoid scratching).
It is the last two, that particularly affects certain classes of trains, especially if the glass coating or film contains microscopic particles of metal. I have no idea what voyager windows are coated with, but I suspect it is a coating including metal.