That's when you physically call your sales rep and explain why your using their services and ask them to whitelist you. I say this from experience. If you called my employer (and any of our competitors, I'm certain) and said "I need to host a mail relay that's ready to go from no traffic to tens of thousands of emails per second, here's my company website, and here's our credit card", we'd be happy to make it work for you.

I'm not saying this to advertise but to offer a suggestion: call someone and ask for help. This isn't an unusual need at all and any reputable provider will be quick to help.

Short version: you don't have to "warm up an IP" if you do a little advance homework.