Yes, it can be done and I am assuming the other PBX is also an IP PBX.
In the ring group, enable the destination to be to a dummy extension that has call forward always enabled. The call forward is set to custom number and the extension of the ring group on the other PBX.
You then set up an outbound route using the peer trunk on the remote PBX that has the dial pattern needed to reach the other ring group. You then setup an inbound route with the same DID pattern and point that to the desired ring group. You may need to disable the forward message on the S PBX if you want the caller to have a seamless transfer. What you are trying to do is to create a path between the PBXs whereby when the dummy extension forward the call to custom number 3000, for example, there is an outbound rule for the per trunk created to reach the other PBX whose dial pattern is 3000 and at the remote PBX there is an inbound route for the peer trunk that looks for the DID pattern of 3000, which you can then direct to anywhere you like. The key is to make certain that the dial pattern used does not match and existing extension.
You may run into CID issues as you may lose the CID of the original caller as only the dummy extension CID may show. You might be able to adjust this using the CID settings in the trunks, but you will need to make test calls and use Wireshark to examine the messaging to see in what field/header the original CID is located, it at all, and then adjust on the receiving PBX, if possible, so that it knows where to get the original CID instead of the dummy extension.
I understood the question and the answer is as I have stated it above.
If all the agents are busy, then you can't transfer it, the system has to do that, which is why you set the fail-over destination to go to the dummy extension, which the dummy extension with the call forward always sends the call to the remote PBX ring group.
Unified Communications
