I did what I think you are trying, but I did it with Obi202's and a
Voip.Ms account.
The customer wanted rollover like old business lines. If line 1 is busy, send the call to line 2. If both busy, send to voicemail, etc.
Although you can configure the Obi to forward a call on busy, I finally implemented the following:
They moved a number to
Voip.Ms.
I created 2 extensions (Subaccounts) and linked each one to an Obi202 Phone port.
I created an extension number on each subaccount for each dialing between phones.
I then configured the DID to connect to LIne 1. The failover said that if Line 1 was busy, go to Line 2.
Although I only did 2 extensions this or perhaps a ring group might suit your needs.
Using the phone switching offsite from the Obi's allows you to do failover to voicemail if none of the phones are connected (such as a power failure or internet outage).
As I say, the Obis do have a forward on busy function though.