I have a Penns Woods push-pin call, that mounts to a gun barrel if I wanted to. I know push-pin calls are thought of as beginners calls, but the sound is pretty realistic, especially up close. I do OK with mouth calls, but sometimes I like to try something different. I'm carrying mine in my vest this spring, just for giggles.
I took a quick look online, looks like Quaker Boy is the only "production" call that clips to your gun these days, but there are a number of smaller companies making them also.