HIi @ErdYar 


The Phone Icon on the Microsoft Teams Application is basically used for making PSTN Call from the Microsoft Teams Client to a PSTN Number.


In case you are able to add folks on the Microsoft Office 365 as a guest users. You should search the user account from the Microsoft Teams Application using the Email Address or using the First Name and Last Name and make a call to the specific person. 


Can you please check on the Below Screenshot if the users are able to see this options? On the Right Hand Top Corner. 



In case they are missing this option then your Office 365 Admin has disabled the VOIP Calling for the Tenant and he needs to enable the same for the Guest Users post which they would be able to make the call to and from their accounts.


Permission/Features Assignment to Guest Users 



With Regards,

Satish U