If you receive a call from abroad that you weren't expecting, you should be careful: it's often a scammer.

Mobile phone provider Vodafone has introduced a spam warning system to protect cell phone users from phone scams. If a Vodafone customer receives a call from a dubious number, the message "Caution: Possible fraud!" will now appear on their smartphone display, the Düsseldorf-based company announced.

The customer can still accept the call, but is warned by the message on the display. "The spam warning system raises awareness among mobile phone customers on the Vodafone network and effectively protects them from harassing and dangerous calls," says Michael Reinartz, Head of Innovation at Vodafone Germany. "It is now automatically activated." The warning will also be displayed for customers of other mobile phone companies that use the Vodafone network, such as Freenet.

For its warning system, Vodafone draws on a constantly updated database containing questionable phone numbers. These include hotlines that intrusively advertise products and services, even though they don't have the consumer's consent required to make the call. The callers also include outright criminals who seek personal information during the call or demand money for flimsy reasons.

However, Vodafone's fraud number database does not claim to be complete - since fraudsters often change their numbers, it may also be the case in the future that a call from them does not trigger a fraud alert and Vodafone customers are not warned.

Such a system isn't entirely new; users can activate corresponding warnings on Samsung smartphones. If a Vodafone customer has a Samsung phone and the corresponding feature activated, they will only receive Samsung notifications and not Vodafone's. The two systems use different data pools.

The telecommunications company recommends that customers generally be vigilant when dealing with new numbers on their displays, especially if they have foreign area codes. If the call only rings briefly and the Vodafone customer then calls back out of curiosity, it can be expensive. Furthermore, customers should not disclose personal information, for example, if callers request such information for alleged competitions or surveys. Vodafone also warns that those called could trigger expensive subscriptions by pressing buttons.

And what are Vodafone's competitors doing? Deutsche Telekom announces that its American subsidiary T-Mobile US is already implementing anti-scam services (scam means fraud). "The company is considering this for Germany," says a company spokesperson. "So the SMS firewall could be followed by a scam firewall in the foreseeable future." O2 Telefónica is taking a similar approach. An O2 spokesperson says they are working on their own measures to warn customers about fraudulent calls.