Imagine the work which would have been involved if what you are saying was true. They'd have had to dig underground ducting in to everybody's garden. How did they do it without us noticing?
You are right in saying that the bulk wiring - the connections which feed the telegraph poles - are now pretty much all underground. There aren't the masses of overhead wires which there were when I was a boy. The final connection to the houses though for the most part remain unchanged. Yes, new builds are all done using underground connections, but most houses are not new builds.
It would be phenomenally expensive to go around replacing all the final connections with underground ones. They haven't done it, and they aren't going to unless there's a separate reason to do it. Indeed, the poles in our road were all replaced recently, in quite an impressive operation. They carry both power and telephone connections to the houses. To replace a pole they had to disconnect everything at the top, temporarily support all the connections with a crane/platform thing, remove the old pole, fit a new pole, and then re-attach everything to the new pole. Now why would they do that if the connections were no longer in use, or even if they had any plans ever to put them underground?
they generally don't bother properly disposing of the telegraph poles: they just let them fall down.
ROFL. Can you imagine the trouble they'd get into if they did that? The ambulance chasers would have a field day.