If you can build the capability of launching a telephone pole that far, you could cause a lot more damage by launching an equal weight of explosives and/or incendiaries instead. If you're going to the trouble of making a hidden pneumatic launcher in a box truck, you might as well make the payload as destructive as possible.

But it might even be easier than that - use augers to target the data lines. I will admit I’m not sure whether such lines are typically underground, but that should be a matter of public record, no?

Incoming power lines won't be buried and will be easier targets. Especially the transformers -- oil-cooled power transformers (as would be typical for that role) can easily be disabled by a single hit from a decently powerful rifle, which will punch a hole in the casing, allow the oil to leak out, and result in the transformer overheating and failing.