No, you can NOT make a minyan over the phone. They must be physically present in the same space. See Orach Chaim ch 55 for the details.
Saying Amen is a different matter. If you know someone has said a bracha, even if you didn’t hear him, you should say Amen. You’re not yotzei anything, but you are simply acknowledging the bracha.
In my opinion making a “virtual minyan” is a wonderful thing, since it gives chizuk and promotes the feeling of davening with a tzibur, but there is no actual tzibur. Therefore there is no need for ten; any number of people davening together over Zoom gives the same feeling, and thus makes our davening better.
Also, it shows the Eibershter that we’re doing what we can. Just as someone who has no matzos for Pesach should use something else to stand in for them, with the understanding that he is not actually performing the mitzvah but only a zecher, so a virtual “minyan” is a zecher to the actual mitzvah that we can’t fulfill today.