Just replaced the base unit of our BT8610 setup. Of course, there are no instructions for how to replace a base unit without losing everything. I just guessed that you have to de-register a handset from the old base unit, then re-register it with the new. I did that in the expectation that it should keep all the settings, phone book entries, etc. Instead it lost the lot.
More amusing was one handset that got part way through the re-registration malarkey and then demanded the PIN. This was a mystery (since we never set a PIN) until I used a handset it *had* managed to register, to discover various old handsets still registered (that is, not ours), a number of address book entries for someone's Italian friends, and a number of call records for during early December (I bought this base unit in late December). After deleting the spurious handsets (there is a limit to how many you can have) it was able to register our final one.
So not only was this unit not new, no one had bothered to put it back to factory settings before sending it out again.
TSC is Telephone Service Centre.