With Congress poised to extend a raft of tax breaks, consider this: One such break has helped AT&T Inc. and Verizon Communications Inc. slash their recent tax bills by billions of dollars without leading to the intended increase in investment or jobs.

The measure, known as “bonus depreciation,” lets companies offset their income with investments they have made more quickly. It was enacted in 2008 as part of the economic stimulus package with the goal of giving companies an incentive to build more factories or upgrade more...