Using a MiCollab account for that would be the cleanest. You'd burn a Standard UCC license, but then you can have either 8 or 16 EHDU extensions assigned and the on-call staff can log in via the web client and switch the device. Set up an availability status for each on call member, and they just need to log in via the web client and change the status. They could even update the EHDU number there if there was a staffing change or someone had to send calls to an alternate number on the fly.
An uglier solution would be to set up a DISA number (with an account code on it) and then dial speed call DN's that in turn dial the forwarding FAC and associated numbers.