I need help with my Iphone.
If you go to Settings > Phone, there is an option to announce calls. With this option, calls should be announced using the SIRI voice, as Voiceover announces calls regardless of whether this setting is activated or not.
That didn't work for me with Voiceover on Siri won't announce it.
I suppose the only option is to sit Siri as your default voice for VO. That way, when there is an incoming call, it will be Siri's voice who will announce it for you.