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Yes.You need cash credit to cover all outgoing and incoming calls and outgoing texts.
Membership allowances don't cover calls and texts while roaming in the USA.Data and MMS will not work.See herehttp://www.48months.ie/membership-plans/going-abroad
You seem to be using an iPhone in which case you might want to be aware of this issue with iPhones while roaming due to the stupid way that Apple do things.
Hope this helps.