SBC Communications Inc.’s planned purchase of AT&T Corp. is still awaiting government approval, but it already appears to be reducing competition in a way that may cost taxpayers.

Before the acquisition was announced in January, SBC was aggressively and prominently preparing to bid for a mammoth contract to provide federal agencies with a wide array of phone and data services, a deal that could be worth more than $20 billion over 10 years between the two winning bidders.