Regarding the time difference you might think of offering freelancers the option to highlight when they ARE available for calls by setting up a) Calling times b) Their current desire for calls.

I.e Bob might be looking for a job and is willing to accept calls between 12pm and 1pm, but not at 3am.

Or Jane may already have enough leads and does not want to accept any calls (too busy) but normally is available between 3pm and 4pm.