I once overheard a conversation between two men, one of whom wondered why native English speakers are such poor linguists. Even when you laboriously demonstrate the pronunciation of a simple foreign word over and over again, they still can't copy it correctly, he said. He went on to warn against ever agreeing to give out the spelling of the word in question, that would permanently prevent its correct pronunciation.
Yes, said the other. Babies who are being encouraged by their parents to say the universal "Mama" and "Papa" ask "How do you spell it ?" and get it wrong, hence Mummy and Daddy.