I would love to see a feature where the iPhone vibrates when a call is answered by the other person. This would be helpful for users who are walking or holding their phone away from their ear while waiting for the call to connect. The vibration would provide a subtle yet effective notification that the call has been picked up, reducing unnecessary screen-checking.
This feature could be optional and toggled in settings under Sounds & Haptics > Call Feedback for users who prefer it.
This post seems more appropriate for the Apple Support Community. The Apple Developer Forums are for questions about APIs and features intended specifically for developers.
You may also wish to file an enhancement request asking for special answering announcements on the caller side. You can file an enhancement request using Feedback Assistant. Once you file the request, please post the FB number here.
If you're not familiar with how to file enhancement requests, take a look at Bug Reporting: How and Why?