Ever since i was little and til this day I've always called my mum "Mummy" and my dad "Daddy". Just because they never responded to "mum"/"dad". And it's always stuck, I suppose. Same with my siblings, too.
I swear every time my mother calls me at work, and i answer saying "hi, mummy", I get the most strangest looks from my work colleagues. They always find that hilarious and usually take the mic. And I can definitely see why it's funny lol
But, it's never changed, and I doubt it will. Mum and Dad will always be Mummy and Daddy. And mainly because they won't respond to anything else from their children!