Voice over Internet Protocol is a steady growing market. It saves a lot of money for providers as well as for users. However, spammers could also transmit their messages over this service. This so called Spam over Internet Telephony is more difficult to filter than e-mail spam, because its content is not available in advance. Two kinds of Spam over Internet Telephony can be differentiated: Spam over Internet Tele-phony via Proxy and Direct Spam over Internet Telephony. Direct Spam over Internet Telephony is the direct transmission of unsolicited messages to a lot of user equipment. Therefore, this is the most dangerous form of Spam over Internet Telephony, because no proxy is passed, which could be able to filter the message flow. A solution to face Direct Spam over Internet Telephony is still missing. This gap is closed by the proposed caller pre-validation mechanism. The basic idea is to get more information about an incoming direct call. Only calls of existing participants are connected. The caller is trusted if his existence can be validated successfully. So, a first technique against Direct Spam over Internet Telephony is given with the presented mechanism.