Not just volunteer work may be forbidden. There is also minimum wages among a large number of laws you have to abide in order to legally maintain any employee, and probably even more if they are overseas like I am. Some employers may also be morally impeded or wary of social consequences of "exploiting" workers even if they agree to it.

Both parties must actively want the other's offerings for work to make sense. If they say they are not hiring, there's no point checking any further whether this may be the case.