Siemens I-SURF - Manual (Page 47)

8. I-Surf USB Utilities
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4. Watch the messages in the status window showing the current status
of the test connection and the test result. Status messages are given
in text form and as CAPI or ISDN error numbers.
5. If the test was successful, the following message appears:
If the loopback test was passed, you can assume that your ISDN Adapter hardware
and software are operating properly.
If the loopback test was not passed, this could be due to one or more of the
following errors:
Error in ISDN access wiring (inverted wires)
Incorrect plug connections
Loose connections within jacks
Terminal resistor pack missing
Incorrect D channel protocol settings
Incompatible ISDN provider D channel protocol
Incorrectly configured MSN
MSN incorrectly configured by the ISDN provider
For additional information please consult the loopback testing program error
messages and the CAPI or ISDN error numbers.