All of those companies have an average price point in mind for a specific service and for bundles! Even in the promotions, the cost over a period of time will not deviate to much from company to company in a geographic area.
While they may break ranks in terms of pricing from time to time, to try to gain business from the other company, for the most part they will tend to follow the leader.
You can bet they study customer trends regularly regarding how long someone might stick with the company or switch service companies. Since there are only a few competitors, it is fairly easy to understand/anticipate their competitors actions/reactions in terms of pricing.
Here is a primer on oligopolies and price setting.
Oligopoly Diagram | Economics Help