KareninaMinnesota,
We know you reviewed the troubleshooting we provided and none of the situations applied. Since noticing the issue, have you had a chance to quit the FaceTime app and restart your Mac? We know that often times we offer troubleshooting that is very specific to an issue, but in a situation like this, sometimes that restart just refreshses everything and resolves situations like this. If not, please do so and test.
If so, you had asked about the relevancy of answering the call on your iPhone. That can be relevant, on the chance, there is a Focus or something enabled that might be preventing answering. It would be more of a contact/account related isolation step. If you're able to connect via FaceTime on your iPhone, we know the situation is specific to your Mac.
Thanks.