A system may provide trust relationship information for a telephone number, where the trust relationship information may indicate whether one or more nodes consider a target node as including or as being associated with a VoIP call agent for the telephone number based on demonstrated knowledge of a PSTN call. The system may determine a target trust relationship to the target node based on the trust relationship information, where the target trust relationship indicates a level of trust a validating node has in the target node being associated with or including the VoIP call agent for the telephone number, and where validating node and the one or more nodes may be in different respective administrative domains of a network.