I work for a multi-national bank one of the largest in fact and we do the same.
The best advice I can give you to avoid having your cards blocked when you're travelling overseas is to ...
... Tell your bank where you're travelling to i.e. departure and arrival dates, which countries you're visiting and be as specific as possible, give them dates.
They will record this information on your account for the fraud departments reference.
Now if they suddenly see your card being used in Belgium or Canada and you told them you'd be there, they will know its you and generally not block your card unless they have reason to believe that the merchant used is suspicious or actually under investigation by VISA/MASTERCARD etc.
Also you should call their 24hr line, they should have access to unblock your card so long as you can confirm the transactions they've flagged on your account are valid.
If you already do all the above and they still block your card then perhaps some of the merchants you've transacted with are questionable.
There is a big list of vendors which are under investigation or have a history of complaints and not matter what you've told your bank if they suspect a transaction they may allow that one to go through but block any further transactions until you validate them.
Hope that helps somewhat
