This would have been my response:
"Hello? Who is this? Do I know you? I think you must have the wrong number. You speak as if you believe you're my mother, but you can't be since my mother died 2 and a half years ago - she was brainwashed to death by a manipulative, divisive, family-destroying cult that worships an organization. Please do not insult the memory of my mother by pretending to be her - you're a cult fanatic and nothing remotely resembling what a true mother is supposed to be. Please do not call me again or I will lodge a stalking/harassment complaint with the police!"