Strange Tone Problem/Not sure where to look

We have an Asterisk 1.4.18 install with FreePBX, about 10 SPA942 phones and SIP connections to both VoicePulse and Broadvoice.

Periodically, say once a day, a call is interrupted by a very long tone, as though someone pressed a number key and didn’t let go. The tone doesn’t disconnect the call–you can stay on the call as long as you can stand but it doesn’t seem to drop.

The circumstances for the tone seem to be as follows: connecting to a webex conference call (universally as soon as you enter your attendee ID the tone starts), getting transferred by some PBX’s (DHL was the most recent one we had problems with but it varies), and even talking to a person on a cell phone (happened three times to me yesterday and all three times the cell phone caller had called me).

Because of the varying nature of the circumstances, it’s hard to figure out what might be the trigger, much less how to stop it.

Anyone have any ideas or suggestions?

I posted this months ago when we were having problems with and a couple of people suggested it was the conference call, but since then it’s been happening too frequently to be a problem with the remote end.