The companies urged customers in the region to keep phone calls short or send text messages, which take up less space on the phone networks.
Hurricane Katrina damaged some communications networks, particularly in the city of New Orleans.
Verizon Wireless said it had resolved earlier call volume issues in the run-up to Rita by adding mobile broadcast tower equipment and reconfiguring its network to increase the number of calls it could handle in certain areas.
"We've expanded the capacity of our network to support the higher call volumes in areas where people are being evacuated to," said Verizon Wireless spokesperson Sheryl Sellaway.
Cingular Wireless, the biggest U.S. mobile service, is the wireless venture of SBC Communications and BellSouth Corp. Verizon Wireless, its biggest rival, is a venture of Verizon Communications and Vodafone.
A representative of Sprint Nextel, the third biggest U.S. wireless service, said the company was not seeing any evidence of network congestion in the region.
Hurricane Rita was expected to make landfall early on Saturday near the Texas and Louisiana border.