Within minutes of a 5.6 magnitude earthquake that hit the San Francisco Bay Area, the number of mobile phone calls on the Verizon Wireless network skyrocketed.

Twenty minutes after the 8:04 p.m. quake, instead of the normal 300,000 calls made between 8 p.m. and 9 p.m. in one area of Santa Clara County, the call volume soared to 2.3 million. Many of those calls were probably made by people trying to check on friends or relatives who lived in the vicinity of the temblor.

And many of those calls never got through because, as often happens after a major emergency, the huge number of phone calls overwhelmed systems that weren't built to handle such high demand. Instead of reaching their destinations, the calls received fast busy signals or messages saying that all circuits were busy.

The incident raises the question: Is this acceptable service? Or can the system be fixed so that every call can go through at anytime, no matter how many calls are being made?

It's not a yes or no answer.

Yes, it could be done, according to wireless carriers, but it would be expensive, and would lead to an overbuilt network that's needed only a few times each year.

"It's not appropriate or feasible," said Chuck Hamby, a spokesman for Verizon Wireless in Florida, where hurricanes are a fact of life and frequently cause problems for mobile phone networks. "You could build, at least theoretically, a network that has enough capacity for everyone in the United States to get on the phone at one time, but [the required switching facilities] would be the size of the Empire State Building."

Instead, wireless carriers build specific cell site buildings that can handle the capacity that's needed in each individual area, and that capacity can be increased as needed, he said.

Usually, those capacity needs are met because the wireless carriers monitor the call volumes to keep up with normal demands. But when a natural disaster or some other unusual incident occurs, demand peaks, causing the call congestion that hit the Bay Area. "Only in truly unusual events do you see this," Hamby said.

If mobile phone systems were overbuilt to handle those occasional but huge spikes, more cell towers, which are controversial in many communities, would need to be built a lot and the added expense would be passed on to customers, he said. "That 60% overcapacity would then sit idle probably 364 days a year" until it's needed in an emergency. "It's probably not the best use of the resources."

Verizon said its systems in the Bay Area worked fine when the earthquake occurred and that all systems were back at normal performance levels 30 minutes after the event. None of its mobile phone towers were damaged or lost power because of the quake, the company said.

The "circuits are busy" messages callers receive are the expected response when the system is overloaded and worked just as designed, Hamby said. "Don't worry. Nothing's broken. It's just too many people trying to get through a revolving door at once."

Heidi Flato, a spokeswoman for Verizon Wireless in Northern California, said no company can economically or realistically build a stout-enough network to handle situations such as the call spikes that came in after the earthquake. "In general, we build for the type of usage we see on a day-to-day basis," Flato said. "To build for that sort of need, for that sort of circumstance, it's like building a second [San Francisco] Bay Bridge just in case the first one falls down. It's just not feasible."