Auerswald COMpact 5200 Advanced Information page 498

Settings – Réseaux publics
Voix sur IP (VoIP)
Interval for NAT Keep-Alive enabled

After the time specified here has passed, NAT Keep-Alive packets are
sent to the Firewall to maintain the NAT mapping.
Note: Some VoIP providers block accounts if the Interval for NAT
Keep-Alive is too short. As a rule, this is reported with error message
503 during SIP registration. If you encounter this problem, we
recommend you set a higher value (e.g. 180).
Proxy sortant
The outbound proxy is an intermediate server that processes all VoIP
requests and connections going to the provider (except for registration).
automatique
The PBX automatically determines an outbound proxy.
Manuel Port
The outbound proxy is a fixed default setting. Enter this data as an IP
address or a URL.
Note: The Synthèses > Ports page shows an overview of the PBX ports
(incoming and outgoing).
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COMpact 5200/5200R/5500R - Firmware V7.2 - Advanced Information V04 07/2018