There seems to be plenty of people complaining about the lack of mobile signal, however I’ve had trips on Voyagers where I’ve streamed music, video and work calls over the full journey from Leeds to Bristol (minor blip around Worcestershire). This has not been the case for a while, and I’ve struggled to get a signal a lot of times in the post couple of years.
That leads me to believe that the problem is with the network rather than the train. If the quality of signal has deteriorated but nothing on the train has changed, the problem must be the network. MVNOs are probably being squeezed out by Virgin Media who now own the O2 network. Tesco, GiffGaff etc are being deprioritised by O2, thus creating connection problems for many people.
That said, reception for all will improve without the window costing.