I do 50 000 calls per month and need to keep proof of calls for at least 5 years.
TNG's call capacity is memory based.
TNG can be configured to use up to maximum of 90% of this memory to store calls. 10 meg can store 25 000 calls. The default is set to 50% of the memory available.
On the TNG login page is will display the period for which it can store calls and when the next auto purge will be.
If you do 50 000 calls a month, you need 20 meg for each month of the year. To make calculations easier take 256meg per year. For 4 years you will then need 1 GIG.
I would recommend a dedicated TNG server with 2 GIG memory. You can allocate 70% for TNG's call capacity. The rest of the memory is then availble for the operating system and processing (which includes TNG processing of for example pulling reports).
Note:
Once 99% of call capacity is reach, TNG will perform an auto purge to bring is back to 80% call capacity in use. The oldest calls will be purged.
You have the option to archive reports. You can set this up in the schedule option under the Report page.
If you set TNG to log rawdata permanent. These files are also available to load into TNG to pull historic reports. Once call capacity is reach the oldest calls will be purged again. Note that if you load a set of older calls in your TNG, it will cause the display on your login page to use the oldest date to calculate the period for which you have calls in TNG (even though there will be a gap in the data date range).