Verizon Wireless, AT&T, Sprint, and T-Mobile. If AT&T were to succeed in buying T-Mobile then that would reduce the number of wireless companies to three. However the size of AT&T would become so large after the acquisition that only Verizon would be capable of competing with them. This means that eventually Sprint will cease to exist, or be bought up. Effectively creating a wireless duopoly in the United States.
Aside from millions of jobs that will be lost, the consumer will be greatly affected if the duopoly were to happen. Firstly the cost of wireless plans will increase greatly. Currently AT&T and Verizon are the most expensive wireless companies, however they have to maintain a reasonable price because if they did not all of their customers would defer to either Sprint or T-Mobile. However in the case of such a duopoly those two companies will be able to dictate the price of wireless plans as they see fit. Secondly the consumer will suffer from a lack of choice, with 2 of the 4 major wireless companies gone choice will also be gone.
Lest we all forget something of this nature has happened once before. What is now know as AT&T used to be known as Ma Bell which served as a monopoly from 1877 to 1954, however a federal mandate broke the company up into several companies. Nobody wants monopolies and duopolies, they are not good for the country and they are not good for consumers.
In hindsight its seems nonsensical to have not known that this could occur. AT&T merged with BellSouth, and Cingular and seems bent on dominating the US wireless market, or at least returning to their former Ma Bell days.