Additional team and channel-related features and issues within Microsoft Teams for Education
Hi Beverley,
Thanks for your updates.
According to the screenshot you provided, it shows that the external users are chatting with a Skype for Business user. However, you are using Teams not Skype for Business. Given this situation, I suggest you contact your Microsoft Teams admin or Microsoft 365 admin to go to Microsoft Teams admin center and check whether the coexistence mode is Teams only or Island. To check it, please sign in to Microsoft Teams admin center with an admin account and navigate to Org-wide settings > Teams upgrade. After switching to Island or Teams only, it may take some time for changes to take effect.
If you still encounter the problem, welcome back to contact me.
Regards,
Seven