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test sending empty UDP packets to SIP phones. "Our telephony systems support STUN and can trigger payload establishment with a provider-side media gateway. (This is needed if the ITSP wants to send payload before we start sending, e.g. if the provider wants to play back an announcement like “number not available” before the local phone starts sending payload.) Furthermore, the rport mechanism is always used (if not deactivated in the ITSP profile), because it does no harm and can help in diagnostics. To keep NAT/firewall bindings open, empty UDP packets are sent to any SIP proxy and registration gateway of all registered ITSP. . . . The filter behavior is of no interest to the HiPath / OpenScape telephony system: It always assumes that it needs to open a binding to the IP address and port of its peers and thus send empty UDP packets to open connections (RTP) or keeping bindings alive (SIP registration). " source: http://wiki.siemens-enterprise.com/wiki/Network_Configuration_for_VoIP_Providers |