If your account is eligible for One Number, you might see this message if you don't have the latest software update on your iPhone, and you've turned off VoLTE (4G HD Calling). You'll need to update your mobile settings before you can sign up for the One Number Wearable Plan.
How to fix it
1. Update your phone's operating system
On your phone, go to Settings > General > Software Update. If you have an available software update, you'll see the option to download and install the update.
Make sure to download the update over WiFi to avoid using mobile data. Before installing the update, you may want to back up your phone.
2. Make sure that VoLTE (4G HD Calling) is enabled on your phone
On your phone, go to Settings > Mobile > Mobile Data Options > Voice & Data. Choose 4G, VoLTE On. Once you've updated your VoLTE settings, restart your phone. Learn more about VoLTE
3. Check SIM settings (Dual SIM users only)
If you have two phone numbers on one phone, make sure the mobile number that you want to use with One Number is set as the Default Voice Line and for Mobile Data.
You'll also need to turn off the line that you're not using for One Number. Go to Settings > Mobile Data. Then tap the plan you want to turn off, then tap Turn Off This Line. Learn more about using dual SIM with an eSIM
Remember, One Number is not available for corporate accounts.