You don't call Amazon, they call you. (So it's free).
Log into your account.
Click 'Help' (in the grey bar near the top of the page).
Select the most appropriate looking option (or use the search facility).
You should eventually reach an option to speak to someone, where you'll be asked to provide your phone number and indicate whether you wish to be called straight away or later.
If you select the former option you'll get a call (from Ireland) within a minute or two.
It's always worked for me!