All of them. Email2Phone places calls to mobile, landline and VoIP numbers in any country.
A minute of call time costs 1 credit to a US or Canadian number and 1 to 6 credits elsewhere, depending on the destination. The calculator on the pricing page gives the exact rate for any number; billing explains what a credit is.
Dial-in, for hearing messages again, uses access numbers in the US, Canada and the UK. Email2Phone charges nothing for dial-in; whatever your own phone charges for the call to the access number is between you and your carrier. From other countries you can still be called; dialling in means calling one of those three numbers.