1) 8 port(8 SIM Card) wireless (GSM or CDMA) VOIP gateway. 2)support self billing,account balance ... -S504 is an analog GSM Cellular VoIP Gateway that helps directly interconnect a SIP-supporting VoIP network with GSM networks. Function: 1,Support ... /1900MHzCall Control ProtocolsSIPVoice CodecsG.729A, G.723.1, G.711 a/ulawNetwork ProtocolsIPv4, TCP, UDP, PPPoE, DHCP, DNS, NAT, Telnet, HTTP ...