People have no issue discussing all types of issues just for fun. Why is it that whenever someone brings up a Torah type of discussion there is such pushback? Are you really afraid that the OP or visitors will turn here when it applies to them practically? Have fun, and discuss your Svaros with whatever sources you can conjure.
Although, the only actual source that came to mind implies, as I mentioned above, that you should not call back, I imagine that circumstances beg for a way to inform them not to come. So, pretty much this is a situation where Poskim would work to figure out a way.
I mentioned Amira Le’akum. But in that case you are being Mach’shil the dispatcher to pick up the phone for a non-emergency in order to stop other people from committing a worse Melachah.
On second thought, what happens every time Hatzolah puts out a call and then scales back, they don’t announce on Shabbos that they have enough members? They obviously do. But then again, Hatzolah itself has Halachos that are based on סופו משום תחילתו. Can that really apply to non-members?