Hi @TK,
Are you using direct routing?
If yes, to me I suppose the issue may be with the direct routing configuration.
Please use the self-help diagnostic in Microsoft 365 admin center to see if it can return cause of this issue.
If it doesn't help, to further troubleshoot this issue, please consider contacting support in Microsoft 365 admin center>Support>New service request and also involving your vendor for help.
Thanks for your understanding.
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