(Reuters) -AT&T ( T ), Verizon and T-Mobile said on Tuesday that some customers were having problems with calls made to other wireless carriers, prompting the Federal Communications Commission to investigate.

The extent of the problem was not immediately clear.

AT&T ( T ) said in a statement: "There is a nationwide issue that is affecting the ability of customers to complete calls between carriers. We are working closely with Verizon to determine the nature of the issue and what actions need to be taken."

AT&T ( T ) added that its network was "not experiencing a nationwide outage."

Verizon said in a statement its network was operating normally but that some of its customers in the U.S. Northeast and Midwest were "experiencing issues when calling or texting customers served by another carrier. We are continuing to monitor the situation."

Tracking website Downdetector.com showed more than 2,300 outage reports as of 5:46 p.m. EDT, with the New York City borough of Brooklyn, Chicago and Philadelphia among the most reported locations.

T-Mobile US said it was "not experiencing an outage. We are seeing Down Detector increases with other providers, so this likely could be challenges our customers are having connecting with users on other networks."

The FCC and New York Attorney General Letitia James said in February they were investigating the cause of an AT&T ( T ) outage earlier that month as well as the telecom firm's response. The outage lasted more than 10 hours and impacted more than 70,000 customers.

AT&T ( T ) credited customers a full day of service for the carrier's outage.