What is the best path to follow to upgrade from Unity 4.0(2) to Unity Cxn 7.0 other than upgrading to 4.0(5) and make use of Cobras.
That is the only way if you want to "upgrade" Cobras is a tool that allows you to export certain data from Unity 4.x, 5.x and 7.x and import to Unity Connection. There a few things you will need to rebuild, but the Cobras help file and videos will explain this more in detail.
My advice, since you are 4.02, is to move right up to 4.1. Im not a huge fan of 4.05.
I have outlined my procedure a couple times here in the forums, but here is the short.
Pull a drive
inplace upgrade to 4.1.x
Retest everything in Unity. If it all works for a couple days, 4.1.x is good to go. If you are not comfortable with it, or had issues during the upgrade, shut down, flip the drives and use your spare to go back to 4.02.
I believe in running 4.1.2 for a couple days before moving on so you are not dragging over any other issues to connection.
Now, with 4.1.2 running, use Cobras at your leisure. Create a test account, export it with cobras, import to connection, etc.
I do not recommend bringing over the voicemails if you can. It's messy and takes forever and a day. Plan accordingly and take precautions with the old messages from Unity 4.x when you do this. It will mark them all as "new" in connection after import.
Id leave the old system for a couple months to employees have access to it, then shut it down. Give them enough time to move off wav files they need or for you to do it for backup if required.