ZyXEL P-2002 ATA series supports standard SIP, which is suitable for IP telephony service deployment and addresses the needs of users who are connecting to emerging SIP-based communication networks. The P-2002L allows users to maintain PSTN phone service and VoIP service at the same time. When ATA power down occurs or VoIP service is not available, outgoing phone calls will be relayed to PSTN automatically. When VoIP service is active, users can place calls to a PSTN phone by dialing pre-fix numbers. Emergency calls or important calls can be relayed to PSTN automatically.