If possible, I suggest you break it up into sections of no greater than 10,000 numbers... I doubt many other dialers could support 1,000,000 numbers at a time.
IVM also has other dialer software (Express Dial) that specializes in outbound calls and probably can load more calls than IVM (see http://nch.com.au/dial/index.html).
Note however that it is designed for humans to handle the calls (e.g. telemarketing), not an automatic computer message system (e.g. book overdue notice) like IVM