Strange tone before placed call begins ringing?

Ok, another mystery with this new phone -- Virgin Mobile LG Optimus Slider. When I place a call and hit dial, there's a weird (and quite shrill) tone, almost like the tone on an answering machine, that sounds before the call starts ringing.

What is this & how do I make it stop, if possible?

Thank you!

 

There's no such option.

 

Nope, I checked there, too.

Call settings only has: North American Dialing (checked), International Dialing (greyed out), and TYY mode (TYY off).

Sound settings only these: Silent mode (unchecked), Vibrate (only in silent mode), Volume.
Incoming calls: Phone ringtone.
Notifications: Notification ringtone.
Feedback: Audible touch tones (checked), Audible selection (unchecked), Screen lock sounds (unchecked), Haptic feedback (checked), and Emergency tone (off).

In the meantime I've asked a friend about this, and he said that when your phone is being tapped, whoever is doing it is required by law to "make you aware" that you're being recorded, and that is done by sounding the "recording" tone prior to your call connecting. Is that legit? It's a brand new phone and it's not like I'm doing anything I shouldn't be.