My Panasonic phone also only holds 30 numbers. Picked up a call blocker from www.procallblock.com that stores 1500 numbers for extra peace of mind since this caller called me from multiple phone numbers despite my not knowing the person they're looking for!

I HAVE AN UNLISTED NUMBER, THIS NUMBER CALLED AND LEFT A LOUD BUZZING SOUND ON MY ANSWERING MACHINE, THEN AFTER THAT THERE IS A RINGING PHONE ON THE  ANSWERING MACHINE, I BLOCK THE NUMBER, THESE PEOPLE ARE SO IGNORANT THEY CAN'T LEAVE A MESSAGE

I just got a call which I will add to my block list.  I will now get no ring but will get a list of blocked calls on my phone.
I have the Panasonic cordless telephone  Model kx-tg6842b (2 handsets)
You can block 250 numbers plus kill the one ring that existed on earlier models that only allowed 30 blocked numbers.
The one handset model sells for $70 at Amazon.