There are some countries which are always there (ie: france, italy, australia, usa..) but you have also a bunch of countries which are on the free list temporarily.

You might still call Costa Rica cheaper than BT though via Voipbuster, but it might cheaper to get one of those prepaid calling card. Just ask at your convenient store and tell them where you want to call.

For example, telco gold is very good for Mexico, but very bad to call Germany or Spain.

I am sure Telco or other prepaid card have special card for Costa Rica

All the best.

I am not mentioning skype here as call to mobile are in my opinion very expensive, however Pc to Pc call have a very good quality.
Check here: