Cell phone companies are the most rapacious and greedy of consumer services companies. There is no ethical quandary about using the loopholes the companies themselves create to make use of free calls. This is particularly true when the internet is involved, since tax money was instrumental in the creation of it.
I think a more pointed question about ethics in telecommunications is why does it cost so very much to send text messages? The underlying technology is basically a mathematical rounding error that costs the cell phone providers nothing. The unethical telecom oligarchy deserves no sympathy from defenseless consumers as they move inexorably toward high priced tiered calling plans, wildly overcharged data plans and the destruction of any semblance of net neutrality.