There is another thread in the google help forum that has a solution to this problem.
The thread is titled "call recording on by itself"
Specifically look at the posts from March 29, 2010 by Scotty Karell
I have some follow up posts as well.
I made the changes suggested in that post and have not had the problem happen at all since then.
I suggest that you try it and post your results on that thread.
Here is repost of the main portion of Scotty's post: (edited and emphasis added by me.)
"I'm willing to bet that most people who are having this issue (like myself) are using one of the Linksys ATAs.
There's a known bug in the Linksys PAP and SPA firmware that emits DTMF tones during a call...
there's a setting that can remedy this issue but may limit your ability to use other services provided by your VSP directly via your ATA: i.e. *98 for voicemail... you would have to manually dial in your whole number (###) ###-#### +voicemail pin to access your messages.
Under the Line 1 tab, change DTMF Tx Method to INBAND