@vanKraster , Based on my understanding of your scenario description, you are trying to use only one ACS Identity and User access token thus only see one participant in call.
Kindly use individual access token for different devices. Every access token represents a user.
If the issue still persists, Is this ACS to ACS or are you trying to bring Teams? We would need more details about your requirement.
Note: Please do not share any PII data on the public forums.