After my conversation with Openserve, I've narrowed it down:
1) If your line is currently working and connecting you to the internet, you're not in the holding pool and it's likely that CW/Evo retains it. You'll have to ask them to put it into the holding pool if you want to leave them.
2) If your line is currently not connecting to the internet, you're in the holding pool now. You can call up another ISP and get them to manage it for you.
3) If you follow 2) and the ISP says they can't claim the line, tell them to call up the infrastructure provider, in this case Openserve, and get them to figure it out.
4) If your line should be in the holding pool, but isn't, and no-one can figure out who owns it, get your chosen ISP to call up Openserve and work it out. Those cases are anomalies, and need to be dealt with on a per-case basis.