David of Broadway wrote:

Most of the NR system is outside the zones, and most NR stations --
the ones outside the London region -- don't have and will presumably
never have have Oyster pads. Oyster pads on the trains would be
useful for people traveling between points outside the London region
and points inside the London region.

Given how easy it was to fit telephone points to new stock (for the brief
period between the technology becoming possible and mobile phone use
becoming so widespread, thus rendering the telephones obsolete and leading
to their removal), it should be entirely possible.