return error reason here.

[zaycode]

It would be better to provide an error code and error message in ZegoUIKitPrebuiltCallController/ZegoUIKitPrebuiltCallInvitationService instead of just a boolean. For example, when calling user B and user B is currently not logged in or logged out, it appears as a bug when the call doesn't appear and there's no error displayed. so we can show to our user the reason error

this example
ZegoUIKitPrebuiltCallController().invitation.send(
invitees: [ZegoCallUser(call.id.toString(), call.name)],
isVideoCall: true,
).then((value) {
print("Message ${value['message']}")
});

[yoer]

This is a good suggestion, and we will consider which version to insert this feature into (but definitely not so fast, because it involves unified modification of all APIs, and the fastest will be next major version, possibly 5.0).