asterisk users - Mar 2003 - SIP rings on after voicemail answers

I have asterisk set up to ring a local SIP phone (on an ATA186) for 
incoming calls and to divert to voicemail after 20 seconds.  However, when 
the 20 seconds is up, asterisk answers (I know this because I have another 
phone on the same line as asterisk) but the SIP phone keeps on ringing for 
a few cycles.  How can this be?  Can it be stopped?  I'm running last 
week's cvs.

  Iain