Make sure you enter a Call Forward All number under the floating extension. I believe the purpose of the floating extension is to be able to forward calls externally or internally without using a seat license. For now, I would view a floating extension as a forwarding number for someone without a physical ip phone.
If you want the floating extension to point to its mailbox, set the Call Forward All number to the Voicemail Access Extension number. Also make sure you setup the floating extension's mailbox before forwarding calls.
As Marcus said make sure the CFNA is set properly. IF you are going to a VM make sure that a VM box was created for that floating extension in CUE. If there is a VM box make sure that it is set to be able to log in from any phone, this should be set by default if created through CCA.
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