Is Wifi calling feature available on ATT prepaid plans?

Who ever told you that is wrong.  It's been available on prepaid since 2017.   Most likely wants to sell you postpaid to get the credit.  

You must have a compatible phone.  Either iPhone 6 or newer, AT&T Android, or one of the few unlocked phones At&t certified. 

If you were told no, it's phone related.  Are you bringing your own?  It's likely a BYOD isn't compatible with AT&T at all. Must be on this list. 

Unlocked phones that will work on AT&T now that 3-g is going away

Yes, on monthly plans, not on pay per use. 

And you must have a phone that AT&T considers capable or compatible with their system in order to use AT&T Wi-Fi calling. If you can't set it up, it's likely your phone is not compatible

Thanks for replying.

I went to ATT store and rep told me that the Wifi feature is for Postpaid only and none of the Prepaid plan has wifi calling feature. 

Its very confusing...

https://www.att.com/support/article/wireless/KM1063258/

Thanks for quick reply, Liz!

I have iphone 8+ purchased via ATT and I am an existing ATT postpaid consumer. My phone is compatible. I want to move to Prepaid as Postpaid plans costs 2x as compared to Prepaid.

I think ATT should mention about WIFI Calling feature in the Prepaid plan description same way T-Mobile does.

Thanks!!