I couldn't find any 3rd party library either. But I recently achieved this by using the flutter_webrtc package, firebase and some native programming, in the following way, roughly:
Sending the call
On iOS I used APN (Apple Push Notification) and PushKit to push a custom message to my app that I parse to start an incoming call through CallKit. On Android, I use FCM (Firebase Cloud Messaging) and FirebaseMessagingService to start my activity showing an incoming call. I haven't had time, but I should use ConnectionService to behave properly with other calls on Android.
Starting the call
As mentioned I use flutter_webrtc for the peer-to-peer video call, but some negotiation needed to take place before the webrtc call could be established, a.k.a. signalling. I use Firebase database for sharing that information between the peers.
I know this is probably not what you were asking for, but maybe it helps! :)
/Saad