Need to force close the "Phone" (Dialer) app manually
Thanks @RossC, for your comments.

Adding this as answer:


  Leave the memory management to the OS. It depends on the version
  (4.1.2 in your case), the OEM (LG) and the particular dialer app. As a
  rule, task killers degrade performance, and the app being in memory
  has no impact on your battery or performance. I'd say leave it be.