Microsoft said the collaboration will allow access to Alexa via Cortana on Windows 10 PCs later this year, and via the Cortana app for iOS and Android at an unspecified point in the future. Similarly, users will be able to access Cortana on Alexa-enabled devices like the Amazon Echo, Echo Dot, and Echo Show.
iPhone users should eventually be able to open the Cortana app and say "hey Cortana, open Alexa," to access Amazon's virtual assistant. Alexa can be used to shop and manage orders on Amazon, ask a variety of questions, stream music and audiobooks, check the news and weather, and many other tasks.
However, iPhone users won't be able to say "hey Cortana, open Alexa" from the Home screen, or when the device is locked. Apple only allows third-party virtual assistants to function within their apps, so accessing Alexa via Cortana will be a lot more cumbersome than simply using Siri.
For a complete virtual assistant experience, iPhone users could also say "hey Siri, open the Cortana app" and then say "hey Cortana, open Alexa."
Alexa can already be used in the Amazon app on iOS [Direct Link], while Cortana has been available on the App Store [Direct Link] since December 2015.