Optimum Voice in NJ

Hi,
I'm using Opt Voice in NJ and I noticed that once in a while, I start to
hear 3 tones, like someone pressing 1, 2, 3 (or some other combo that the
octave climbs), then I get dropped.

It even happened when I dialed into the local voicemail number to pick up
messages.

Does anyone know what could be causing this? Internet, TV (with HD) are
fine.

Thanks

Mike