Netopia Network Adapater User Manual page 43

Select ENABLED to enable STUN (default) if the VOIP ATA is behind a NAT enabled
router and the router has no ALG for SIP, or DISABLED to disable STUN (VOIP ATA is
not to use STUN for NAT traversal). VOIP ATA also supports a proprietary
implementation of NAT traversal where the Service provider is expected to provide some
relay support. If DISABLED is selected, then based on the responses received, the VOIP
ATA will dynamically determine if the SIP Server supports the proprietary
implementation.
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