The post service is called Correos, and they have a website you can translate into English. http://www.correos.es, it is good for working out postal charges and also for tracking parcels, I had to email them at one point to find out what happened to my parcel and managed that no problem, got reply in English. I have used the postal service for delivery of items bought and sold on ebay, the charges are similar to the UK.
If you want to phone home when you are on holiday it is best to use the internet cafes, you use the phone then pay after the call and it very reasonable,and most of the time a lot more private than the phone boxes on the street.
The internet cafes are also good for the internet, or printing boarding passes, or photo copying. Now it has become popular for bars and cafes to offer free wifi, which a lot of the younger generation use.