Random Touch Tones sound off during calls?

We've been aware of this for about a year now, and I'm finally going to write about it.  We have Comcast Business voice, and we've noticed that during our conversations with clients, periodic "touch tone" type sounds are played over our line.  On our end, we only get a small fraction of a second of the tone, but according to our customers, they hear a full volume touch-tone in their ear, as if we pushed a touch tone button inadvertantly on our handset.

I can guarantee you we did not, at many of these calls are done hands-free with speaker phone and we're no-where near our phone.

Also, if our phones somehow were doing this, I believe we would hear the full duration of the tone, like our customers do, not this abbrievated short version of the tone.  It doesn't bother us, because it's so short, but our customers have complained.  So, now, I just have to take action and find out what's doing this.

Does ANYONE else out there have a similar issue with this?  It feels to me like it must be coming from Comcast.

Happy to be proven wrong, as I just want to fix it wherever it is.

Doug