stick someone else's simcard in it, then call
Not someone elses sim, but another provider, any operator is okay (even from another country), if you get to the regular screen (not stuck when starting), then you are okay.
When talking about a blackberry phone, you might need to UNlink with blackberry, via your provider.
If they don't want to (you are still on contract or whatever), then go to your operators site, and look for the roaming prices in different countries.
Most operators have partners in roughly 200 countries. Just pick a country which is NOT on the list, and tell them that you are traveling to that place (staying there for 3 month), so you will need to get a local providers blackberry service, or you aren't 'hooked on' any longer...