Android contacts showing up on my iPhone

Hi all

The other day I helped set up my dad's new Android phone. A couple of days later, all his contacts were showing up on my iPhone, and my contacts on his phone.

I didn't log into any of my dad's accounts like gmail etc on my iPhone, nor did I log into any of my accounts on my dad's phone.

I can't figure out why the two phones' contacts keep syncing - any advice would be great!

Thanks.