No, if your iPhone is dead or powered off it will not ring. To ensure that you do not miss calls, make sure your iPhone is always charged and turned on. You can also enable a setting to have incoming calls forwarded to another number when you are unable to answer the call. To do this:
1. Go to Settings > Phone > Call Forwarding
2. Select “Enable” and enter the phone number where you want all incoming calls redirected
3. Calls will now be forwarded if your phone isn’t answered or if it's switched off