travelling and calling abroad

can I use my phone abroad? calling abroad how much does it cost to call abroad from the UK? how much does it cost to use my phone when I'm abroad? will texts I send whilst abroad be taken out of my bundle? I have previously called to activate roaming. Do I need to do this each time I travel? how do I make a call abroad? how do I contact customers services when abroad? how do I call a telephone number in the UK whilst I am abroad? are there any bundles that'll make the calling abroad charges any cheaper?