Zoho Calendar on iPhone

Hi, i'm new to Zoho, just joined today! I only want to use it for my calendar, and i've set this up fine on the web browser and also integrated it with Thunderbird. But, when i add it to my iPhone (using the Exchange method described by others on the forum) it constantly gives me a password error. I've tried resetting my password, but no joy. What am i doing wrong? Many thanks, Mark