Phone Description to Regarding Case Workflow - Append with problem

I have a workflow that takes the information in the Phone Call Description Field and updates the Regarding Case Description Field when I convert the phone call to case. This works. What I also need it to do is... if another phone call is taken and is set to regarding the existing case to append the existing Case Description Field with the "new" Phone Call Description, and not over write the existing content.

I have the first property set to:

{Description(Phone Call)}

The second property is append with:

{Description(Regarding (Case))}

--{Description(Phone Call)}