This would be a tier2 ticket, tier1 can make the initial ticket and look to escalate it to tier2.
This seems to be an issue when the internal O365 email user is missing the SIP address.
We don't manage clients exchanges.
Also when creating a Skype Meeting you can't add presenters that are external users.
All Troubleshooting steps / How To:
o Notes: If this issue is presenting itself when trying to add internal users then most likely they are missing the SIP email address.
Follow the fellow steps or give them to the exchange admin to do in order to fix this.
o The error message would look like this.
o Solution is to: Have the client Exchange Admin go to:
O365 Admin Portal >Exchange Admin >Recipients >Mailboxes > Select User > Email Address Tab >Add New >Type SIP
> add the email address of the user > Click OK > Click Save.