Hi @Ouk Choi
Is the Skype for Business user is in your organization or the external user?
For Skype for Business, did they deploy the bybrid environment?
If it is external user. please check if the contact user's domain is allowed in Microsoft Teams Admin Center > Org-wide settings > External access.
The coexistence mode also has impact on your situation. If the user in your organization is in Islands mode, chats and calls from Skype users to Teams users are delivered to Skype for Business.
For more information about the requirements and limitations of interoperability between Teams and Skype, please read this official document.
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