There are a lot of cheap radio controlled toys (cars or others). What thay basically do is they just activate power at the other end.
You may try to use one of these systems to radio-activate your dog's collar device ..
If you like twiking about, then take a simple dog wisle and see with a scope what the frequancy is. Then find ultrasonic speaker ( from an old remote or from a supplier ) that can prosuce the same requeancy as the wisle. Then you can buy off the shelf a remote door bell for you home that uses RF and use that to attached the speaker to the dog. Its the fastes way that i can think that you can build it and use it...
Good luck.