Telecom stocks tumbled Thursday, posting declines while the overall market was slightly positive.

Here's where the major telecom stocks stood as of Thursday's market close.

AT&T down 1.2 percent to trade at $31.01

Verizon down 3 percent to trade at $59.08

Sprint down 6.1 percent percent to trade at $5.66

T-Mobile down 4.3 percent to trade at $69.15

For comparison, the and the tech-heavy Nasdaq Composite Index were both just slightly positive as of Thursday's close. Verizon, Sprint and T-Mobile are still trading in the positive over the past 12 months, with shares increasing 23.1 percent, 16.5 percent and 13.6 percent, respectively. AT&T has remained in the negative over the past year, down 12.8 percent.

The dip among telecom companies may be due to reports of potential setbacks in Sprint and T-Mobile's merger approval process. On Thursday, Bloomberg reported that Sprint has privately told regulators that it will not be able to compete effectively in the market if the merger is blocked. Its argument has been met with skepticism from regulators, according to Bloomberg.

A spokesperson for the Federal Communications Commission declined to comment on the merger. Sprint did not immediately respond to CNBC's request for comment.

If the deal between Sprint and T-Mobile is approved, the telecom market would slim down from four to three major wireless carriers. While critics worry this will harm competition, the companies argue they will be able to use their combined power to help ready the U.S. for faster 5G networks and compete more effectively with the top two players, AT&T and Verizon. Narrowing the field to three players could lift the industry overall by increasing pricing power.

The FCC paused the informal 180-day "shot clock" to review the $26 billion merger deal earlier this month to allow more time for the public to comment on the deal, according to Reuters. The FCC said at the time it typically takes this action after merger applicants submit "substantial" new information and said it expects the process to resume in early April.

Correction: Bloomberg reported Thursday that U.S. regulators are skeptical of Sprint's argument that it would not be able to compete effectively with other telecom companies if the merger was blocked. An earlier version misstated the day.

Subscribe to CNBC on YouTube.

Watch: 5G will change your phone and your world