The Public Utilities Commission of Texas is asking for the public's input on a planned 737 area code overlay for the 512 area code.

The overlay would apply to the entirety of the region currently covered by the 512 area code, which includes all of parts of Bastrop, Bell, Blanco, Burleson, Burnet, Caldwell, Comal, Coryell, Fayette, Guadalupe, Hays, Lampasas, Lee, Llano, Milam, Mills, San Saba, Travis and Williamson counties.

Having the overlay would mean users would need to include the three-digit area code in addition to the seven-digit phone number when dialing within the region. According to a news release from the PUC, the agency plans for a six-month period of either seven- or 10-digit dialing from December 2012 through May 2013, and mandatory 10-digit dialing would begin in June 2013, with the 737 area code being activated the following month.

PUC spokesman Terry Hadley said the PUC holds the meetings to address concerns and answer any questions phone customers might have. He said the overlay is not a foregone conclusion, but the other options are not as attractive.

One of those options would be to have a geographic split, which would, for example, compress the 512 area code to a smaller geographic area and have the 737 area code for the other parts. This is what happened in San Antonio when the 830 area code was introduced.

"The problem with geographic splits is invariably, some customers have to give up their numbers to join a new area code," Hadley said. "That is an inconvenience we find most everyone likes to avoid. With the overlay, everyone gets to keep their existing number."

The PUC estimates 512 phone numbers would run out sometime after late 2013. Once the 737 overlay was put in place, new numbers could have either the 512 or the 737 area code.

Hadley said the transition should not saddle customers with any additional costs, which is another reason why the overlay has become the preferred option. He noted older alarm systems might need to be programmed to dial out with 10 numbers versus seven, but he said newer models should already be programmed that way.

"(Area code) 737 has been in place for quite some time because there was the thought about 10 years ago that the new area code would be needed much sooner," Hadley said.

He said by partnering with telephone providers and using a process called "number pooling," the PUC and the providers were able retrieve more numbers and delay the transition. Within an area code, there are 792 available prefixes because some are reserved and some with zeros can not be used.

"Each prefix has a maximum of 10,000 available numbers, so the most numbers you could have within one area code is about 7.9 million," Hadley said.

Originally, the 512 area code covered all of South Texas, roughly from the Austin metro area to Corpus Christi.

The PUC public hearing is scheduled for May 17 at 9:30 a.m. in the Commissioners' Hearing Room on the seventh floor of the Travis Building, 1701 N. Congress Ave.