Outbound calls disconnectd by long tone

We have a strange problem with two different phones, an SPA941 and now a 942.

While making certain calls from the handset through Asterisk, we will hear a long tone that essentially disconnects the call. You can leave the phone off-hook for a while and the tone continues. One day it happened on three attempts to call the same number.

In the past it only happened while calling a conference call set up by freeconferencecall.com but it is becoming more and more common. I am not sure if the numbers we are calling that trigger it are also VOIP numbers or not.

Any advise or suggestions would be greatly appreciated.