In a oligopoly like the phone industry, it is illegal to collude (agree to fix prices at a certain point), because that is allocatively inefficient (Price is not equal to marginal cost - the companies are making more money than they should compared to consumer surplus). This is why we have anti-trust laws. However, it is natural that prices would end up being similar or the same. If one company raises their price, customers will just go to their competitors. If a company lowers their price, then customers will go to them, so their competitors will lower their prices as well. However, this will mean all the companies will just lesser profit, so they don't end up lowering their prices.
