Certain peoples voices trigger the DTMF sound. VoIP works differently than POTS, and frequencies are picked up by the VoIP adapter and converted into a DTMF sound.

I get that once in a while with my wife (but only when her voice gets raised, like she's yelling at the kids).

I think adjusting the volume on the vPanel can help. There's another thread on here recently that talked about this.

-Craig