New York, New York - Nearly 75,000 AT&T customers reported cell phone service outages Thursday, according to tracking website Downdetector.

The issue was clustered in several cities, including Dallas, Houston, Chicago, Atlanta, and Miami, the website said.

The number of outages rose suddenly at 3:00 AM EST and spiked further after 6:00 AM EST, according to Downdetector.

AT&T advised customers to use Wi-Fi calling while it works to restore wireless service.

"Some of our customers are experiencing wireless service interruptions this morning," the company said. "We are working urgently to restore service to them. We encourage the use of Wi-Fi calling until service is restored."

Downdetector additionally indicated that users on T-Mobile, Verizon, and Cricket Wireless were reporting a smaller number of outages.

However, T-Mobile said in a statement that its network was "operating normally" and that the reports were likely "reflecting challenges our customers were having attempting to connect to users on other networks."