NEC has announced new technology to counter the growing problem of unsolicited VoIP calls, known as Spam over Internet Telephony (Spit).

The company's Seal software checks all incoming VoIP calls to see whether they are likely to have come from a human, rather than software, based on communications patterns observed during the call.

Seal uses modules to identify particular call segments to be blocked at source, thereby improving the efficiency of VoIP systems. NEC will be showing off the software at 3GSM this year.

Spit is a variation on standard automatic diallers that are banned in many countries. Calls are made announcing bogus lottery wins or prizes, which can be collected by calling a premium rate number.

However, the growth of VoIP has made the unit cost of such calls much lower, and experts are worried that Spit will harm the overall take-up of VoIP communications systems.

"Spit has the potential to completely ruin VoIP. No one is going to install the system if they're going to get dozens of calls a day from audio spammers," said security guru Bruce Schneier.

"Or, at least, they are only going to accept phone calls from a white list of previously known callers."