I have a customer with an SV8100 that is spread across six buildings all on the same property and there are several employees that could be anywhere on the property at any time. Being that there is lots of outdoor area, a cordless solution isn't really possible. Aside from call forwarding a user's desk set to a cell phone every time they get up, is there a way to get the desk set and cell phone to ring simultaneously and then if the call isn't answered, for the caller to end up in voicemail for the desk phone?

I've looked at Mobile Extension but not sure if that suits the bill. Looks more like a stand alone way to make the cell phone look like a separate extension.

I tried Tandem Ringing with the digital set and an analog extension but forwarding the analog extension to the cell phone does not work.

Thought about using a department group with the digital set and a forwarded virtual but then not sure how to make the voicemail function properly.