Military cancellations and number transfers

If I use the military cancellation and hold my number while unlocking my phone, can I have my number transferred to a international carrier who supports the region I will be in during deployment?

Not how it works.

Either you're going to have AT&T hold your number or not. If you transfer your number somewhere else it's canceled off of AT&T service.   And you can't transfer a number if it's not active.

You also cannot transfer phone numbers to carriers in different countries. American numbers stay in America. Canadian numbers stay in Canada. If you're being transferred for example to South Korea, you cannot transfer an USA number and use it domestically in South Korea 

You may be able to submit deployment orders, get your phone unlocked, and transfer to Google voice but you have to make sure you time it very carefully. If AT&T puts the number into cancellation before you transfer it it's not available to port to a virtual service.