The grid run their own lines. Underground power cables are always accompanied by a pilot cable, which used to be 2x current loops plus a bunch of twisted pairs, now fiber optic. Underground power cables are almost always run in pairs (pairs of 3 phases) so between every significant substation there's always at least 2 dedicated communication lines.
You have said 'owned by the telcos' and this is very much not my experience. The grid comms network is owned and maintained by the grid.