Reuse of Interaction IDs:
How a call ID is generated/recycled: the Call ID ? or Interaction ID, as seen in the client is composed of a single digit (1-3), followed by the three-digit Site ID, then followed by a six-digit sequence number (000000 through 999999). So the first digit will cycle from 1 to 2, from 2 to 3 and then from 3 to 1 once the last six digits run through their cycle. The Call ID is meant to recycle itself after this process. In looking at the example below, we believe your Call IDs are recycling every 3-4 months which would be in line with your call volume, etc.
The Interaction ID Key (which I know may not be available in all ICBM searches/queries, will be wholly unique. In IC 4.0, the Interaction ID Key is an 18 alpha-numeric field, consisting of the ten digit Interaction ID usually followed by an eight digit date (YYYYMMDD). Switchovers, allocations of very large numbers of interaction Ids and restarts may further modify the eleventh digit of Interaction ID Key to make values unique. The eleventh character may be mapped to 'A' and will be advanced to 'B' and beyond if additional switchovers occur. For example, suppose that the Interaction ID of an interaction is 1234567890, and that the date is August 14, 2007. The resulting Interaction ID Key would typically be 123456789020070814, but the eleventh digit (the "2" in "2007") might be changed to keep the string unique.