3Com does not have anything that will "merge' in CDR records from different systems. However, there is a partner of 3com, Trivium that has a CallAnalyst program. You should contact them. (www.triviumsys.com)
As for the purge interval, it is really dependent on the number of calls and a matter of disk space. The purge interval can only go as high as 60 days. It was just that customers will download the data and keep it on their PC.
So, just a matter of disk space which is not an issue with the newer NBX'es (was with the older NBX 100's with the smaller disks)