Casa is compatible with both iPhone and Android. However, due to how keys are stored on your device and backed up to the cloud, it's not currently possible to retrieve the same Mobile Key and Single Key on a different phone when you're switching from iOS to Android, or from Android to iOS.
The Mobile Key and Single Key must be restored on the same phone type (iPhone to iPhone, or Android to Android). This is because these backups are synced to iCloud (if you're using an iPhone), or Google Drive (if you're using an Android phone).
"What happens when I set up the Casa App in a different OS?"
When you're setting up the Casa App on your new phone with a different OS, you will need to generate brand new Mobile and Single keys. Generating these keys in a different OS will generate new keysets on your phone.
"How do I access those funds?"
To access the Mobile and Single Key funds, you will need to restore the Casa App on a different device with the same OS that you used to first back up the device.
If you still have your old device with the other OS, you could use that device to sign the mobile key portion of the transaction you make to your new multisig keyset.
"What if I don't have the old device, and don't have access to a phone with the same OS?"
If you do not have your old device, and do not have access to a device with the same OS, then you can recover funds from your multisig keyset by initiating a recovery transaction using your hardware device in combination with the Casa recovery key.
You will not be able to restore Single Key funds without a new phone using the same OS.
"Can I opt out of using the Mobile Key?"
Yes! In the Casa App, you can optionally replace the Mobile Key with a second hardware device key.