A server, upon receiving a request to complete a call over a packet-switched network, looks up an address on the packet-switched network on a local table. If the local table does not contain a matching address, the server asks a routing server to identify a receiving server address. The routing server identifies a suitable address and sends a message to the originating server containing the address. The routing server may consider such factors as the volume of calls currently being handled by various potential receiving servers in selecting a receiving server. The receiving server then receives, via the packet-switched network, a message requesting a telephone connection to a second telephone device. Before completing the connection to the second telephone device, information in the message representing the address of the device on the packet-switched network that originated the message is identified. The address information is compared to selected predetermined addresses for permitted origins of the message. The permitted origins may be other servers or clients of the receiving server. If the address in the message corresponds to a predetermined permitted address, the server will proceed with completion of the call. If the address in the message does not correspond to a predetermined permitted address, the server will refuse to complete the call.