Call waiting beep for the person calling?

I haver verizon and when my girlfriend calls me and im on the other line she knows it. She gets this little beep after the ring which means i am on the other line. But when i call her and shes on the other line it doesnt beep for me. Last night i called her and for some reason it did the ring beep thing. When i asked her what she was doing she didnt say anything about being on the other line. So i accused her of it and she just kept denying it. Does anyone know what causes these beeps? And why when she calls me and im on the other line she gets it all the time but when i call her and shes on the other line i dont get it? But i got it that 1 time last night? Im confused. BTW she has sprint.

it's a tough call (no pun intended). Perhaps it may be a feature of sprint and not verizon. I have not noticed that before, but I'll give it a test.

Sorry, I was talking to your girlfriend.

Just kidding, but someone was. That 'little beep' is there to let people know when the person that they are calling is on another call.