asterisk users - Feb 2004 - Problem sip registration

hello

I have a Cisco ATA working with asterisk, but I have east error when it
is tried to validate in asterisk.

Feb  2 10:04:42 NOTICE[1125342512]: File chan_sip.c, Line 5210
(handle_request): Registration from
'<sip:501@216.xxx.xxx.72;user=phone>' failed for
'200.xxx.xxx.24'
Feb  2 10:04:42 NOTICE[1125342512]: File chan_sip.c, Line 5210
(handle_request): Registration from
'<sip:501@216.xxx.xxx.72;user=phone>' failed for
'200.xxx.xxx.24'

My sip.config
[501]
type=friend
username=501
secret=cisco
canreinvite=no
host=dynamic
dtmfmode=rfc2833
mailbox=501 
nat=1

Thanks

Is anyone else noticing high lag on their voicepulse IAX2 connections?  
We're seeing 500-600ms lag time when our pings to gw5.voicepulse.com are 
only 90ms.  Our other IAX2 connections to this box are not experiencing 
anything else like this.  Our server has hundreds of megs of free ram 
and is very idle CPU wise so I don't our Asterisk is bogging down.  Just 
curious if anyone can shed some light on this.

Thanks,
Jonathan