Loud Tone after Mid-Call Disconnect

[chefele]

My parents, who have FIOS digital phone service, have been seeing weird symptoms related to a persistent call cut-off problem.   I don't think it's their phone or wiring.  Here are four major symptoms:

1. While using the FIOS digital phone service, after calling someone and talking a while, the line occasionally goes dead on their end & the call is disconnected.

2. On the other end of the call,  the person they're talking to (not a FIOS customer)  hears a loud tone as soon as the "disconnection" happens. The tone persists until the called person hangs up, even if it's for minutes...it doesn't automatically disconnect their side of the call.  The caller (on the FIOS end of the connection) never hears this tone.

3. If the caller (FIOS customer) then redials the other person while they're sitting there listening to the tone, the other person can receive the new call via call waiting, and then continue their conversation.  

(To me, this symptom #3 seems important ---  if the tone the other person hears stays on even after the FIOS caller hangs up, this seems to mean the tone is not being generated by their phone)

4. So far,  this seems to only happens with long-distance calls, not local calls. 

5. The same symptoms have occurred when calling many different people, so it's not something on the called-person's side of the calls.

6.  Normal hangups work fine, ie when they (FIOS customer) hang up, the other person  never hears the tone & the call disconnects normally.

I have some theories.... But does anyone out there have some ideas about a possible cause, or any troubleshooting tips?  Thanks.