Beeping during phone calls

[Blind Freddy]

When I make outgoing calls my MotoG gtives 2 double beeps every 40-60 seconds. It sounds like call waiting, but is not. It happens in every call I make using VOIP. OK so it's my VOIP app...but I tried another app and still there. So I assume it's the phone. I have asked my VOIP provider and tghery swear black is blue that it's not them. So whether that's true or not I have to look elsewhere.

SO has anyone had this or have ideas about why?

Thanks for any help

Nick