In Bill Stegman's office on the eastern side of Dallas, the phone is ringing and ringing and ringing again, with only short breaks between callers. It goes on for some 30 minutes, virtually uninterrupted. He says it's been like this for hours.

The powerful and deadly tornadoes that have torn through Granbury, Texas, and Moore, Okla., in the last week are fueling this activity. What these dialers have in common is the hope that Stegman, the owner of storm-shelter installation company American Tornado Master, can keep them from being the next victim. He's been in this business 36 years, and he says he's never seen interest this high.

Storm shelter: Credit American Tornado Master

"Not even back in 1999, when Oklahoma was hit [in] that tremendous outbreak then, was it nearly as furious as it is now," he says.

By 11 a.m. Central time Tuesday, Stegman says he had received more than 100 calls about his rooms on the day. Another 193 emails had arrived in his inbox. On Monday, he got approximately 700 inquiries combined. That was the day a massive EF5 tornado killed at least two dozen people in Moore, an Oklahoma City suburb roughly 200 miles north of Dallas.

No price can be put on a human life, a truism that's been made abundantly clear in the past few days. Following the strong storms of late in the central U.S., residents of north Texas appear to have a new appreciation for that fact, and they aren't letting the potential dollar-expense deter them from considering protection.

Price of safety

The cost of a safe room varies widely depending on the seller, the materials involved and the installation specifications -- a buyer has to determine how much defense is required and at what price. The shelters Stegman sells, which are constructed to endure winds of as much as 250 mph, start at right under $8,000. Made of concrete or steel, and with a number of in-ground or above-ground options and sizes, the average total outlay for one of his units is about $10,000 to $11,000. In a typical year, he would expect to sell 150 to 200.

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[Related: Witnesses Describe Deadly Oklahoma Tornado; Gov. Fallin Says Death Count Unknown]

"What we're trying to do is help prevent something like we've seen here in the last week," he says.

In Granbury, six people were killed when a tornado hit the town May 15. The Moore tornado has claimed many more lives, though a final number isn't yet available. According to the National Climatic Data Center, Texas leads the nation with an average of 155 of these rotating storms a year. Residents of the state aren't unaccustomed to violent weather, but that doesn't mean they all have a good place to go, such as underground, in the event of a tornado. For instance, in Dallas County, basements are extremely rare.

The elevated demand hasn't been unique to American Tornado Master. Tim Arp, who works for another Dallas shelter company, ArmourGuard Storm Shelters & Safe Rooms, speaks of a similar spike in contacts for his firm.

"We're getting probably 150, 160 calls a day right now," he says, explaining that concerns about the deadly capabilities of the weather picked up after the Granbury storm. "The next morning is when it really started. It's just one phone call after the other."

At ArmourGuard, the base price for a shelter is $5,000, and that can rise to $15,000 or even beyond. Generally though, its units sell in the $7,000 to $10,000 range, Arp says. His company installed around 120 rooms last year, working mainly in the greater Dallas-Fort Worth area.

How much a family can afford naturally is going to vary from house to house. Lower quotes can be found online, including a $2,895 starting price for a shelter at Oklahoma's Tornado Alley Armor. Should the cost of a safe room be prohibitive, the Federal Emergency Management Agency offers assistance in some areas, administering a rebate program that, if a homeowner is eligible, can partially cover expenses, up to $3,000.

[Related: The Cost of Tornadoes]

Both Stegman and Arp say shelters built for existing homes make up the bulk of their business, with new construction accounting for a small piece of their orders. One thing that has changed is that, over time, the location of their installations has been shifting from a primarily rural base to areas that tend to be closer in to cities. (A video on an ArmourGuard installation can be viewed here.)

"I think Joplin kicked that off, when folks saw that actual cities could be hit," Arp says, referring to the 2011 Missouri tornado that killed 158 people.

No such thing as 'tornado-proof'

Shelters from American Tornado Master and ArmourGuard have been tested at Texas Tech University's Wind Science and Engineering Research Center for their ability to withstand debris from tornadoes up to EF5 strength. During the trials, researchers will use a machine to hurl 2 x 4 pieces of wood at a high rate of speed at a shelter, then judge its performance.

"We've had all of our doors and our shelters tested there, and we've made improvements on them," Stegman notes. Shelter doors in particular have been enhanced from past years, incorporating stronger metals, sturdier hinges and more locks than they previously had, he points out. Guidelines are available that describe specifications against which shelters are measured, with FEMA P-320 and the International Code Council's ICC 500 serving as the industry standards.

Even so, "no one on Earth can guarantee you anything is tornado-proof," he says. "Because no one can say you can withstand the acts of God, and that's what that is, is an act of God. I don't tell anybody, 'you buy my shelter, and I 100% guarantee you you're going to live.' No one can do that and be honest."

For many people, a shelter can be an impulse buy, one driven by news of death and destruction, Stegman says. Once the weather calms down, people may be less apt to commit to a purchase. "But I promise you they will be back," he says. "The tornadoes will be back."