Hello
If in normal operation the CUCM publisher of the cluster is sending properly to the FTP server
the CDRs generated by the cluster, when the server will not be available the CUCM publisher will still try to send them.
As the CUCM publisher detects it is not sucessful sending the CDRs it will keep them. When the server will be back online the CUCM publisher will forward all the pending CDRs since last time it was successful.
Make sure your system is configured to kee the CDRs during the downtime. You can find details in the links below:
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/service/8_6_1/admin/sacdrm.html
Use the CDR Management Configuration window to set the amount of disk space to allocate to call detail record (CDR) and call management record (CMR) files, configure the number of days to preserve files before deletion, and configure up to three billing application server destinations for CDRs. The CDR repository manager service repeatedly attempts to deliver CDR and CMR files to the billing servers that you configure in the CDR Management Configuration window until it delivers the files successfully, until you change or delete the billing application server on the CDR Management Configuration window, or until the files fall outside the preservation window and are deleted.
Both the CDR Agent and the CDR Repository Manager process files with an interval that is independent of the CDR File Time Interval.
- The CDR Repository Manager sends all existing CDR files to the billing application servers, sleeps for 6 seconds before checking the new files to send, and continues that 6-second interval. If the destination (the external billing application servers) does not respond, the system attempts the process again by using a doubled length of the sleep interval (12 seconds). Each delivery failure results in double the sleep time (6, 12, 24, 48,and so on, seconds) until 2 minutes occurs, then stays at 2-minute intervals until successful delivery occurs. After successful delivery, the 6-second interval automatically resumes.
Users cannot configure the 6-second processing time, with the sleep time interval doubling in case of failure.
- The CDR Agent behaves in almost the same manner. First, it sends all the existing CDR files to the publisher. If no additional files to send exist, the CDR Agent sleeps for 6 seconds before checking for new files. Each delivery failure results in the immediate change of the sleep interval to 1 minute, then says at 1-minute intervals until successful delivery. After the first successful delivery of files, the 6-second interval resumes.
If you need to start or restart the file transfer timer for any reason, you can restart the Cisco CDR Repository Manager or CDR Agent process by going to the Cisco Unified Serviceability window and selecting
Tools -> Control Center -> Network Services
.
Note 
Regardless of whether you enable the deletion of files based on the high water mark parameter, if disk usage exceeds the disk allocation that you configure, the CDR repository manager service deletes CDR and CMR files, beginning with the oldest files, until disk utilization falls below the high water mark.
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/service/8_6_1/car/caranrpt.html
Configuring the CDR Repository Manager
The CDR Repository Manager sends CDR files to up to three preconfigured destinations (billing servers) using FTP/SFTP. It also maintains files on disk to make sure the storage usage does not exceed predefined limits. If you exceed the predefined limits, the CDR Repository Manager deletes old files to reduce the disk usage to the preconfigured low mark. Files get preserved for a certain number of days based on configuration. Files that are old enough to fall outside of the preservation window get automatically deleted.
Use the CDR Management configuration window in Cisco Unified Serviceability to configure the following items:
•Set the amount of disk space to allocate to call detail record (CDR) and call management record (CMR) files.
•Configure the high water mark (HWM) and low water mark (LWM).
•Configure the number of days to preserve CDR/CMR files before deletion.
•Disable CDR/CMR files deletion based on the HWM.
To access the CDR Repository Manager configuration window, open Cisco Unified Serviceability and choose Tools -> CDR Management.
I hope it helps
