1) how can I get the ringtone volume of MS Teams on iPhone up?
As of right now I do not see the ability to adjust the ringtone volume of MS Teams nor does changing the volume affect anything..
2) And/or change the ringtone?
You cannot change the ringtone of MS Teams.
I have created feedback to the Microsoft Teams feedback team to track the issue/enhancement: (Inability to Change Teams Volume (on mobile app/phone)
https://feedbackportal.microsoft.com/feedback/idea/d1aa3638-3e1b-ed11-a81b-6045bd7d68ae
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