Basic economics and misunderstandings
The demand curve isn't fixed, but instead can shift. So while it's generally true that a change in price will lead to a change in volumes sold, the reverse doesn't necessarily hold. Rather, telecoms operators are quite aware that demand for their services is growing constantly, partially due to lower prices, partially due to network externalities, service combination, and technological progress. Given this constant increase in usage anyway, operators could well find that to profit maximise they would be best served raising their prices.
Of course, this doesn't take into account the fact that wholesale prices are regulated to fall anyway, so of course things will get cheaper ...