You need to create a new error handler (CCXML or VXML, depending on your app). You can download the current error handlers by logging into your Experience Portal, selecting MPP Servers, then Event Handlers button. From there, you'll see the tabs for VXML and CCXML. Download the current error handler, make your modifications, than upload the new version (it's best to give it a unique name so as not to over-write the default).
Once you've uploaded the new version, you have to set your application definition to use the new error handler. From the EP, select Application, then select your application. Scroll to the last section (Advanced Parameters). Under this section, select your new error handler in the appropriate section.
NOTE: The error handlers are CCXML code. Be sure to test extensively to be sure you're handling things appropriately. CCXML can be extremely finicky.