If you applied inland, I might be worried about asking for an extension since this will show you've been outside of Canada for several week and plan to be outside of Canada for several more weeks. I would lean towards flying back and getting the test done.
Also, the call centre won't be able to extend your medical (they don't have that kind of authority / power over applications). You would have to email the visa office handling your application (in this case Mississauga by the looks of things) to request an extension. But you certainly won't hear back from them immediately. So the best you could do now is email Mississauga, tell them you can't have the test done now, ask for a few week extension and hope this is accepted (of course you won't hear back in just a few days so there's some risk involved with this approach).
