It could have been that the original owners never had a Telco landline, thus only the CATV wire
however, per code, it has to be grounded, ie, you can't just run a wire from the pole to a jack
Sometimes on old CATV installations, it was just a grounding connector on the outside of the house.
If vinyl siding was installed after the CATV wire was installed then, the ground could be under the siding
If you have a basement, look in there.
I have seen Telco interfaces in attics, but those were on old houses.