asterisk users - Feb 2006 - Adjusting frequency asterisk sends NOTIFY's to ATA's at for MWI.

I'm trying to figure out how Asterisk decides how often it will send SIP
NOTIFY's to an ATA when a voicemail message is waiting for the user on the
server.
>From watching, it seems to be completely random.  Sometimes 10 seconds
apart, then 33 seconds, then 13 seconds, etc.  Each time causes a
"ring" on
the ATA.  I can change the ATA's behavior, but what I'd really like to
do is
just modify Asterisk to send a MWI NOTIFY every 10 minutes or so.

Is there a config option I've missed or is this behavior that I'd have
to
modify in the source code?

Ray