For computer hackers and virus writers, the new frontier is your cell phone.

According to computer security experts, a phone virus or “Trojan horse” program could instruct your phone to do some extraordinary things — like call the White House or the police with a bizarre hoax. It might forward your personal address book to a sleazy telemarketing firm or something as simple as eat into the phone’s operating software, shut it down and erase all your personal information.

Similar nasty high jinks have already dogged cell phone owners in Japan and Europe. “If a malicious piece of code gets control of your phone, it can do everything you can do,” said Ari Hypponen, chief technical officer of F-Secure Corp., a computer security firm. “It can call toll numbers. It can get your messages and send them elsewhere. It can record your passwords.”

As cellular phones and even PDAs continue to morph into computer-like “smartphones” able to surf the Web, send e-mail and down-load software, they’re prone to the same tribulations that have waylaid computers over the past decade.

“We should think of cell phones as just another set of computers on the Internet,” said Stephen Trilling, director of research at anti-virus software maker Symantec Corp. “If they’re connected to the Internet, they can be used to transmit threats and attack targets, just as any computer can. It’s technically possible right now.”

In Japan, deviant e-mail messages sent to cell phones contained an Internet link that, when clicked, caused phones to repeatedly dial the national emergency number – equivalent to 911. The wireless carrier halted all emergency calls until the bug was removed.

In Europe, handsets’ text messaging capability has been used to randomly send pieces of binary code that crash phones, forcing the user to detach the battery and reboot. A new, more sinister version keeps crashing the phone until the text message is deleted from the carrier’s server.

In the United States, relatively primitive cell phone technology has kept users immune from such tricks — but not much longer.

Until recently, cell phone operating systems were “closed,” unable to download software. But new smartphones — like the Nokia Communicator, Handspring’s Treo, Motorola’s Java Phone and Mitsubishi’s Trium-Mondo – are open to such third-party downloads.

It’s these third-party programs that worry experts. If one is disguised as a Trojan horse, an infected phone could make some calls on its own. Soon, mobile-phone owners will be obliged to install security software like “personal firewalls” that used to be reserved for Internet servers.

Cell phone users can avoid this, of course, by sticking with their old “dumb” phones, said Alan Reiter, a wireless consultant.