My female would let out a single note chirp as a release call if another frog stepped on her. It doesn't sound at all like a calling male, and is much softer.

As Amy has mentioned, the young males may try to call and it won't sound quite 'right', but there's usually an attempted trill.

In the winter, my male is set off trilling by the furnace kicking in, misting the cage, and if I play recordings of Greys calling. Males are much less picky about the time of year than females are. In the wild I'll hear isolated Greys trilling late in the summer, long after breeding season is done.