This applies to Vigor 2100V, 2500V and 2700V models only.

When you are on a call which is going through your VoIP provider, and there is an incoming call on the PSTN line, the Vigor’s “Call Waiting” feature will play beeps to inform you of another call coming in.

Unfortunately the beep tone interrupts your current call (blocking the voice call while playing the tones)

At the moment the only alternative is to turn the Call Waiting feature off, as described here.