Mess with your coworkers or give your workspace that unique sound to make the rest of your coworkers jealous. 
Or actually use this because your company decided to give everyone must have that unique company ringtone

Before we begin, you need two things

7-zip:  https://www.7-zip.org/download.html
Asar Plugin for 7-Zip: https://www.tc4shell.com/en/7zip/asar/

1. Install 7-Zip
2. Depending on if you installed the x86 or x64 version, go to either C:\Program Files\7-Zip or C:\Program Files (x86)\7-Zip
3. Create a new folder the 7-Zip installation directory called Formats
4. The Asar plugin should be just a zip file with Asar.32.dll and Asar.64.dll move these files to the folder created in step 3

This completes the prequisites to modify the files for teams. 

Now we can start modifying the ringtone(s)

1. You can use anything as long it's in MP3 format. There is no size limit, the lenght is determined by how long Team keeps ringing before it switches over to Voicemail. 
   So get your file ready from anywhere
2. First Teams needs to be closed before you can edit anything, otherwise it will complain that an application is using it
3. Go to C:\Users\%username%\AppData\Local\Microsoft\Teams\current\resources 
4. Rightclick on the app.asar file and open it with 7-Zip
5. Navigate to assets > audio
6. Unfortunately you can't copy your downloaded MP3 file into this folder and expect it to show up in teams. Sadly it has to replace one of the many existing files in that folder. 
   You will have to rename your downloaded MP3 to ring.mp3 so that every thing in teams that uses the default setting will use that MP3. 
   You are free to replace the whole thing as long as the name matches.
7. Start up teams and ask someone to call you to see if your ringtone works
   Sadly you cannot test this on your own, if you go to settings and preview the ringtone it will still play the original one. The custom ringtone only works if someone calls you. 

It is possible to push these changes to many users at once or just one user like for example your co-worker. All you need to do is follow the same steps on his/her computer or remotely by using the UNC path. 
If you do it remotely you will need to find a way to remotely close teams, this can for example be done with PStools PSExec by connecting to the computer and use taskkill /IM teams.exe /F
You can prepare the app.asar file by using the one on your own computer, modifying it and then copy it to another computer. 

Or if you use something like Intune or SCCM with your script to close teams, copy the app.asar file and start up teams to make it look like nothing happend. 

All roads lead to rome...