Some questions on triband and network

Ok,

Is a triband phone supports ALL network around the world?

Can I still use my network (DiGi - Malaysia) in other country in Asia/EU/America)? Or I've to change to the network provider in that particular country?

Thanks in advance for any reply answers!

If your SP has a contractual agreement with a network in another country then you won't need to buy a new chip, when they've got an agreement they will just bill your own network.
As far as tri-band phones goes they don't cover all networks, I know there is one in Brazil that is on a different band to most tri-band phones.

Thanks for the info!