Do I need the RTC?

While the RC210 has its own onboard Clock & Calendar, they are not battery backed up. This means that should the RC210 lose power (or otherwise restart), the time and date will be lost.

 The RTC option solves this problem by providing a battery backed up Clock & Calendar as well as providing additional Message Macro, DTMF and Remote Base Memories.

 Details may be found here

Even after I plugged in my RTC, I still hear "Clock Not Set" when I call the time

Despite using gold plated connectors, sometimes grime gets on the contacts, especially if the controller has been in service a long time without an RTC.  It is NOT a sign that the RTC's battery is dead (if the battery is dead, the time will be spoken as January 1st, 2000.)

 The solution is to power down the controller, remove the RTC from its connector on the RC210  pc board, reconnect it and power the controller back up. This will clean the contacts and allow the RTC to operate properly.