They haven't announced one. My guess is that they're in the middle of complicated negotiations with cell operators at this point.
They certainly seem to have big ambitions for this project – they've supposedly secured 85% of Globalstar's bandwidth [1], and I doubt that calling for ambulances or tow trucks, or users sharing their location manually (once per 15 minutes at monst) requires that much.
My guess is that iMessage over satellite will become part of some of their higher-tier iCloud plans, in line with Apple's "services" ambitions and as a way to test the waters of providing network access directly.
GP is (I believe) replying to an iPhone-specific remark in my original comment.
I've brought up Apple/Globalstar because it serves as a baseline for what's already possible, as well as it being a likely future competitor.
And I wouldn't read too much into (in)compatibility at this point: Neither Starlink nor Apple have disclosed how their respective solutions actually work. Starlink might function as a "dumb pipe" analog frequency-shifting relay just like Globalstar.
