All taxing and subsidization is distortionary; but some methods are less distortionary than others. Imposing a tax on "everybody" and using it to pay for a public service dampens economic activity, but at least doesn't distort one industry relative to another. Imposing special, industry-specific taxes, however, dampens demand and investment in that industry relative to the rest of the economy.
Take tobacco taxes, for example. We put special taxes on tobacco because we don't want people to smoke, and we want to discourage people from smoking. Taxing tobacco effectively raises prices, which reduces demand. Various governments employ similar taxes to discourage things like soda, sugary foods, etc. This is a well-understood concept: you put extra taxes on things you want to discourage (relative to the rest of the economy).
Paying for rural telecom by putting a special tax on telecom service achieves exactly the same effect. It increases the price of service, reducing demand and reducing investment (relative to the rest of the economy, which does not bear that tax). But in theory, telecom infrastructure is something we want to encourage, not discourage.
It's even worse when you impose a general public obligation on individual service providers. For example, instead of paying for universal healthcare with general tax dollars, you could simply direct hospitals to treat poor people for free. Nobody does that--they pay for the broad public benefit with equally broad public taxes. But we do it for telecom--we tell companies that in order to be allowed to offer any service in a city, they must shoulder the burden of building out infrastructure even to neighborhoods where few people could afford to subscribe (i.e., neighborhoods that can only be served at a loss). That essentially makes it impossible to have a "minimally viable ISP." You can't compete with an incumbent by stealing away customers neighborhood-by-neighborhood. If the government simply let companies build infrastructure where it was profitable, and built subsidized infrastructure itself using general tax dollars where it was not, you could have that sort of competition.