I have a iPhone. Can I call from Costa Rica.

These are the countries AT&T prepaid roam in the link below. Costa Rica is not one of them. 

https://www.att.com/prepaid/static-pages/intl-travel-add-on/

Link below has info on prepaid roaming. 

https://www.att.com/support/article/wireless/KM1387490/

as far as i know prepaid doesnt roam in costa rico

@formerlyknownas  correct?

What do I need to do to get coverage.  International? 10.00$ a day?  Or 35.00$ week ly?

 prepaid wont work there regardless of what addons you purchase 

Wi-Fi calling may work when connected to a Wi-Fi source. Have not read where it works in non roaming countries for prepaid. 

What do I need to do to get coverage.

Again, prepaid does not roam in Costa Rica. There is nothing you can do to get coverage. If you have WiFi calling activated, it might work from Costa Rica but no guarantee it will.

I think I answered my own question