When you use Microsoft Lync in Microsoft Office 365 Dedicated, you can't use the simultaneous ring feature in Lync.
This behavior occurs because the simultaneous ring feature in Office 365 Dedicated requires Lync Online Enterprise Voice. If you bought the Lync Online Enterprise Voice service plan, this feature works correctly.
If you do not have a Lync Online Enterprise Voice service plan, you should use the call-forwarding feature in Lync.
For more information about the call-forwarding feature in the Lync client, go to the following Microsoft website: