Password Problem

Back in May, I activated the international service on my Tour 9630.  Since then, the device occasionally refuses to accept my password (required by my employer, who furnishes the BB).  Doesn't happen every time I leave the local network area, but it appears to solve itself when I return.  Any ideas why this is happening?  Our IT group says it must be a network issue, not the phone itself.  Could it be related to the switch to international service?  I'd appreciate any suggestions.