Would you give your Social Security number to the clerk at the grocery store? What about the mom of your daughter's friend? Or the cable guy? Of course not, right? Yet, if you're like most people, you wouldn't hesitate to give out a different, but equally important number to these same people.

Experts now say that your cellphone number may be even more valuable to spammers and identity thieves than any other personal identifier. In other words, instead of handing it out, you should be guarding that cellphone number with your life. Here's why.

Back in the old days, whenever a person moved or even changed phone companies, they got a new phone number. Between college and my early career as a seasonal park ranger, I probably had two dozen phone numbers. But today, landlines are, for the most part, a thing of the past. Instead, most Americans have cellphones, with phone numbers that they will likely keep for a very long time.

That means that if a spammer gets your cellphone number, it will be that much harder to get rid of them. It also means that over the years, your cellphone number will be linked to a treasure trove of personal information — from contact lists to Google searches, to passwords and credit card numbers, all information that hackers would love to get their hands on.

"Smartphones are not just communication devices," said Adam Levin, co-founder of online security company Identity Theft 911 in an interview with Market Watch. "They are data storage devices."

Those 10 little digits open a lot of doors

A few years ago, German security researcher Karsten Nohl did a demonstration on CBS' "60 minutes" to show what he could obtain using only a person's cellphone number. California Congressman Ted Lieu was given a new cellphone for a day and Nohl was given that cellphone number — not the name of the person connected to the phone or even where that person lived. But using just that number, Nohl was able to pinpoint Lieu’s location and movements throughout Los Angeles, read his email and text messages, and record phone calls between Lieu and his staff.

And Nohl did all of this from his office in Berlin using just a cellphone number.

Still think it's a good idea to post your number on a bulletin board?

It's safe to assume that Nohl is an expert when it comes to hacking, but even your run-of-the-mill thief can use your cellphone number to wreak havoc. When you sign up for an account online, whether it's a new email address, or Facebook, or an account with an online retailer, you usually provide your email address and a password. But if you should forget that password, many of these companies will send you a password retrieval code via your cellphone.

Do you see where this is going? Let's say you post an item for sale in an online forum and you include your name, email, and cell number. You've now given a potential thief everything he needs to hack into any of your online accounts.

Do Not Call doesn't cover everything

On the less invasive but equally disruptive end of the spectrum are spammers. Individuals can list their personal phone numbers on the Do Not Call Registry to prevent unsolicited sales calls. But the registry is only good for personal numbers, not business lines. So if you use your cellphone for both work and personal calls, spammers will still be allowed to make those calls, particularly if you've done business with them in the past. In addition, the Do Not Call list only applies to phone calls, not texts. So spammers can text you all they like, even if you're charged by your phone carrier for those messages.

So what are your options? According to Federal Trade Commission analyst Bikram Bandy, it's illegal for businesses to require customers to give their cellphone number to complete an order. However, that may not stop them from making your phone number a required field in online forms. If you want to avoid handing out your number, you may need to call instead of using an online form.

There are also a few apps (such as Sideline, eVoice and Google Voice,) that allow you to access a separate phone line or answering service for your business using your current cellphone. So if you need to hand out your cellphone number to clients or have one printed on your business card, you can use this number while protecting your personal line.

The bottom line is that until the next big thing comes along, your cellphone number will probably be with you for a long time. It just makes sense to protect yourself by guarding that number — and all the access it allows — as closely as you would any other personal information.

Why sharing your cell number isn't a good idea
Your cellphone number now rivals your Social Security number in importance, and you will likely have it for a long time.