I'm test driving Cape right now. I started on iOS and it ran fine. It has some shortcomings on iOS, like group messages with both iOS and android users will be broken apart, no international roaming, no WiFi calling, and no IMSI rolling (brand new this past week).
I've since switched to GrapheneOS which they wholeheartedly support and it works well. If you buy a Pixel from their store, they'll take $300 off your bill over six months, reducing it to $15. They'll also contrbute to the GrapheneOS project. They do not support RCS, but I expect that they assume as a customer seeking such a service that you'll be using a secure messaging service.
Porting numbers in and out are completely painless with no questions asked in either direction. The system is fully automated and you'll never have to talk to a rep trying to upsell you or whatever. If you do port out, plan for it. Once you initiate the port out, it has to be completed within six hours. I did port my iPhone back to VZW, and bought a Pixel 9 Pro XL and put it on Cape with a new number. If you do want to port your number in to the system, you need to do it when signing up. You'll be assigned a temporary number immediately until the automated porting process completes. Your phone will then be assigned the ported number and the temporary number will be returned. If you want to do it later, simply cancel your account and create a new one. I've done this several times between iOS, pixel 7a and Pixel 9. Note, however, unless you're coordinating by his with their support team, its $99 each time. After canceling your account, it remains active until the end of the billing cycle. If you delete the eSim, however, you have a lot more work ahead of you if you change your most and and decide to reactivate.
Their service is still very much in public beta and they're not shy telling you that. So keep that in mind going in.
Their support team is U.S. based and exceptional. My interactions with them were through chat feature in the required companion app that resides on your phone. I do believe there is support through email as well.
The device mention in an earlier post that you may have seen a couple YTers discussing is likely their Obscura product which is not currently available to the general public. It has additional obscurity features not found in the public beta that I'm currently using. But as I mentioned, they did just introduce IMSI rolling in the android product which is a cool feature born from Obscura.
Their blogs and tech briefs are very well written and informative. They discuss a lot of the common questions folks ask about the majority mobile operating systems and carriers.
Well, I typed a lot more than I thought I would! Short story is that in its current state it is a usable carrier that meets its mission of a minimal KYC carrier with unique security technologies on the back end. On the client (phone) side, the features favor android, but they're working to bring feature parity to iOS devices.
I will continue my experiment for the foreseeable future.