A recorded message for AT&T cellphone customers and the company's official Twitter account said customers were having difficulty making or receiving calls because of technical troubles.
According to media reports, there were problems with about 1,000 cell phone towers that were not expected to be fixed until about 2:30 a.m. Sunday.
Most customers have been able to send and receive text messages and use the Internet, it's just making phone calls that has been tough.
It's still unclear how widespread the problem has been.
AT&T offered its customers an apology for the outage.
The Associated Press contributed to this report.