A method for packet telephony includes receiving over a packet communication network (22) a first message originating from a first end-user (32A) in a first aggregating organization (28) to place a call to a second end-user (32D) in a second aggregating organization (30). Responsively to the first message, respective values are assigned to one or more security attributes that are indicative of a likelihood that the call is a spam call. A second message is transmitted over the packet communication network toward the second end-user, the second message containing at least one of the respective values that was assigned to at least one of the security attributes.