We haven't been asked much about bunnies, but had calls mainly about cats.
The 2 stray rabbits we have taken in-
Xena, I'm sure was a stray because the woman who found her threatened to let her go if the vet couldn't find someone to take her. Xena was pretty aggressive so I guess she was dumped!
Mallow, was taken in by 2 people and the story was a bit suspect, they also had 2 female rabbits. We think the girls started to fight, Mallow had wounds on her, and they decided to get rid.
Taking the number doesn't always work, Dumpling, the old cat we took in, was handed in by someone as a stray but when the vets tried to call her it was a false number.