How do I teach my rescue dog to come to me when I don't have food or toys?

How old is your dog and how long have you been working on the recall?

Personally I never completely stop rewarding the recall with whatever the dog's highest motivator is (food or toy) because the recall is the most important behavior for my dogs to respond to and while I may go on a variable or random reward schedule, I like to pay my dogs for coming when called.

I will reinforce it with the ecollar later in their training (hit send too soon)

How long have you had her and how much time have you put into her recall training so far?

since your dog is new to you and she's a rescue I would not be concerned with training without food/toys at this point. Don't call her if you don't have a way to make sure she has to come (leash or long line at all times) and you have a way to reward her.

If she's not on leash and you don't have rewards, don't call her. If you call her and she can choose to NOT come to you (no leash, no rewards) then you effectively teach the dog to ignore the cue.

Also see our online course - Training Your Dog to Come When Called