Mobile phones are expensive abroad, on a par with hotel rates.
I don't have a pay-as-you-go phone, so don't know if there is something specific that will stop it working abroad. Certain phones only work in certain coutries ie only tri-band phones work in Europe and US.
If it does all work in Barbados, I doubt that �100 would last very long. Price of calls vary, but if you work on the basis of �1.50 a minute you won't go too far wrong. If someone calls you when you are there they pay 50P (or whatever the normal cost of calling your mobile is), you pay the remainder. So you will be paying for incoming calls.
The cheapest thing to do is purchase a calling card when you get there and use public phone boxes.