From the bloggers at Outside Interests:
Many banks use fraud-detection software that looks at your purchase patterns and tries to guess if a purchase is legitimate. The problem is that sometimes this software sees you making purchases all over the place as you roam, and your card can be blocked automatically.
Tip: Major banks often have a place on their website where you can report your travels in advance, especially when traveling outside your home country. This is okay if you only roam occasionally.
If you travel a lot, contact the bank and tell them to put a note on your account that you travel frequently. If the bank still repeatedly blocks your card “for your protection,” don’t waste time arguing with them—just switch to another bank that has a more travel-friendly policy. They’re not all the same.
Check out the Outside Interests blog for tips on using credit cards while traveling.