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Always create a case to report the issue and following up with the 24x7 Hub via phone if deemed necessary. This will provide Ex Libris with the necessary info in written form so nothing is lost in translation. The 24x7 response commitment is 1 hour and I have found them to be very responsive within this time frame when reporting System Down issues.
You should let Christina know if you have not received responses within the specified time or if you are unhappy with how your issue has been treated.
More info on reporting system down issues can be found on the Knowledge Center. A FAQ is also available with more information regarding coverage.
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