Snom ONE IP Technical Manual page 157

Because some providers will accept the proposed duration without bothering to
refresh the NAT binding, you need to force the system to renew the registration
in a shorter period of time and ignore the service provider's registration. This is
done through the Keepalive Time setting below.
17. Keepalive Time: Keepalive time is the time required to keep the registration
live before it re-registers. This setting helps with trunks that connect to ITSPs
that have a good session border controller. This setting may cause a lot of traffic
to your service provider, so be sure not to enter an unnecessarily short time.
Typically, this setting can be left blank.
18. Send email on status change: Once a trunk has been registered, it is critical
that status changes are brought to the attention of the appropriate individual.
For example, if the service provider goes off line and the trunk looses the regis-
tration, the system will send an email notification to the email account that is
specified.
19. Strict RTP Routing: This setting is needed because the IETF allows "RTP traf-
fic send ports" to be different from "RTP receiving ports," creating an extremely
NAT-unfriendly situation. While most implementations today use the same
port number for sending and receiving RTP, some gateways still insist on strict
IETF compatibility. In such cases, this setting should be enabled. We recom-
mend you keep this flag disabled ("No") unless asymmetrical ports are required.
20. Avoid RFC 4122 (UUID): Not all registrars ignore RFC 4122 (UUID) when
it does not support it, so it is necessary to explicitly suppress the UUID in reg-
ister requests. SIP implementations that are not able to deal with UUID report
a "Bad Request" to indicate that they were not able to process the request. The
UUID is used to indicate that a registration replaces another registration, which
is useful for avoiding double registration after a system restart.
21. Generate unique extension number: This setting is used for emergency calls,
so that the 911 call center can directly call the extension without having to go
through the auto attendant. When this setting is set on the trunk, the system
will use the EPID of the extension to represent where the call is coming from.
(If it does not exist yet, the system will allocate a new EPID.)
22. Accept Redirect: Use this setting if your trunk should respect redirect codes.
By default, this introduces significant security risks because the system can-
Chapter 4: Trunks 137