Hi @IBN
According to your description, do you mean this issue only happened in external phone number call, right?
Considering this issue is not persist all the time, we recommend you contact with this user to check their caller ID Policy. Based on my knowledge, the admin could set caller ID polices as anonymous user in Teams admin center. For more details about it setting, you could check this article:
Meanwhile, please check if this problem also exist in Teams web client. If there is no issue in Teams web client, try to clear Teams cache file and restart it to see if it could be fixed.
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