MENLO PARK, Calif. - Sheila Vreeburg has entrusted her cell phone number to very few people — her family, close friends and her veterinarian. If she could keep it that way, she would.
After all, Vreeburg, an insurance agent and native Californian, has kept her home number unlisted for decades, and directs her mail to a post office box. Her cell phone number is even more sacred.
This kind of passion for privacy is prevalent in Western states, and that could signal an uphill battle for proponents of a national cell phone directory.
Nationwide, slightly more than a third of Americans have unlisted home numbers, but in California, Nevada, Arizona, Oregon and Washington, about half the people choose not be listed in phone directories. They pay as much as $2.66 a month to keep their home numbers private.
The wireless industry, meantime, has promised it will submit to the 411 directory the mobile phone numbers of only those customers who grant their carriers permission to do so. But to be sure, some federal lawmakers want to mandate that.
‘‘We have to protect the privacy of cell phone users, and we want to have the rules of the game set down here . . . to ensure that cell phone users do not face an onslaught of unwanted calls,’’ said Sen. Barbara Boxer, D-Calif., who introduced the Wireless 411 Privacy Act along with Sen. Arlen Specter, R-Pa.
They’re not just worried about privacy invasion. There’s a pocketbook factor: Cell phone users must pay for many of the calls they receive.
Under the proposed bill, cellular carriers must first get existing customers to authorize the inclusion of their mobile numbers in the 411 database. For new customers, carriers must clearly and conspicuously give them the option to decline.
The bill would also ensure that no customer need pay a fee for having cell phones unlisted.
The Cellular Telecommunications and Internet Association, which hopes to compile the wireless directory by the end of the year, says it considers the legislation unnecessary given the industry’s vows.
In addition, wireless carriers — with the exception of Verizon Wireless, which is not participating out of customer privacy concerns — say they won’t sell the wireless directory to third parties, put it in a printed telephone book or post it on the Internet — any of which would make it easy for phone numbers to get in the hands of telemarketers.
But consumers like Vreeburg are skeptical that such promises would effectively block unwanted calls and unscrupulous advertisers.
‘‘Even though I’m unlisted, I still get telemarketer calls. On a cell phone, it’d be ridiculous,’’ Vreeburg said.
Privacy advocates say it would be in a wireless carrier’s best interest to keep their word — and keep their customer numbers out of telemarketers’ hands.
Unlike their terrestrial counterparts, cellular carriers know their customers can easily flee to a competitor, said Chris Hoofnagle, associate director of the Electronic Privacy Information Center.
The Washington D.C.-based advocacy group supports the federal wireless 411 privacy bill.
‘‘Without baseline consumer protection legislation, privacy provisions can change as quickly as business models change,’’ Hoofnagle said. A federal law passed in 1991 already prohibits telemarketing calls to cell phones, but ‘‘creating a directory heightens the risk of misuse of information,’’ he said.
Despite the reluctance from many customers, the wireless industry contends the proposed wireless 411 directory was born of consumer demand — with small businesses most interested.
Kimberly Kuo, a spokeswoman for the cellular industry trade association, said as many as 10 percent of the nation’s 165 million cell phone users have no land line.
‘‘Like with anything new, it may take time to beef up the directory, but there’ll be certainly plenty of numbers in there, even if it starts with just small businesses,’’ Kuo said.
Still, in California, where the right to privacy is enshrined in the state constitution, lawmakers aren’t waiting for federal protections and have proposed their own wireless privacy bill.
Not all states are as privacy-conscious.
About a third of the households in Michigan and a quarter of those in Arkansas and Wisconsin are willing to pay the $5 per month that companies charge there to keep their home numbers unlisted.
And only about 10 percent of the households in Maine have unlisted phone numbers, according to Survey Sampling International, a Connecticutbased database firm.
‘‘California has always been different,’’ said Linda Piekarski, vice president of research at Survey Sampling. ‘‘But in Maine and Vermont, those are states where people still leave their doors unlocked, and you’ll find that people there are very open and trusting.’’
Sure enough, while Boxer has had her home number in the San Francisco suburb of Greenbrae unlisted for as long as she could remember, Sen. Patrick Leahy and his wife are right there in the phone book, among the 93 percent of Vermont households with listed numbers.