Snom ONE IP Technical Manual page 158

Deploying the snom ONE IP Telephone System
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not determine whether these redirects introduce additional costs (redirection
to expensive numbers). Therefore, enable this setting only if you are sure that
your service provider or SIP gateway does not abuse this feature. This flag is
especially important when you use the system with Microsoft Exchange or other
Microsoft products, such as the speech server. Also enable this setting when you
have a trunk that comes from another system. This will make it possible to call
from one system to another.
23. Interpret SIP URI always as telephone number: When a call comes into the
system, the system needs to know how to interpret the number. In SIP, the URI
contains a domain name; however, in most cases, the domain name should be
ignored when interpreting the URI coming from this trunk (because the user
portion is just a telephone number). Usually, this is indicated by the parameter
"user=phone," but not all service providers set this flag. By turning on the "In-
terpret SIP URI always as telephone number," you make the system believe that
this flag was set on the trunk call.
24. Requires busy tone detection: When an analog PSTN gateway (e.g., FXO)
is used, hangup detection can be an issue. In FXO, the hangup signal might be
just a tone that needs to be detected. Unfortunately, no international standard
exists for the disconnect tone. Incoming international calls might give you a dis-
connect tone that the system has to recognize. Of course, if the PSTN gateway
is capable of detecting this, the task should be left to the PSTN gateway. How-
ever, as a fallback, you may also configure the system to perform the hangup de-
tection. The disadvantage of doing this is that it costs additional CPU resources,
and it might randomly disconnect calls; for example, if the other party plays
back a tone that sounds like a busy tone, the call may be disconnected. The best
way to avoid these kinds of problems is to use a digital line, e.g., a SIP trunk.
25. Trunk requires out-of-band DTMF tones: When this setting is enabled, the
trunk will use RFC 2833.