How do I sync iPhone calendar with iPad?

[iMore Question]

my isp is at and t. I use its gui calendar and have for years with no problem and it syncs nicely with my ipad but doesn't want to sync with my iphone

 

[pkcable]

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I have not heard of gui calendar, is this an apple app store app, or something else? Is it web based? Also are you using the same AppleID on both the iPad and the iPhone? Are you open to using any other calendar programs such as the built in Apple calendar, OR google calendar?