Prior to travelling make sure you contact your network provider and advise them that you plan on travelling abroad to ensure your phone will work once you arrive. I made this mistake once and would not do it again!
Whilst you are on holiday, it is only natural to want to keep in contact with friends and family back at home. However it can often cost a small fortune to ring home from a mobile phone. From personal experience it is not the best feeling arriving back off your holiday, only to find a phone bill on your door step for a stupid amount of money!
If you are on a budget you are probably better off texting people you want to keep in touch with, especially as it is free to receive text messages abroad. If you receive calls however it can cost as much as a pound a minute.

Beware if someone calls you and leaves a voice mail message, you can be charged for picking it up as well as being charged for receiving the message itself.