[asterisk-users] Asterisk forgetting about client registration or Polycom phone forgetting to register?

[C F]

I'm having trouble with Polycom 501 phones that asterisk forgets how
to reach them.
/etc/asterisk/sip.conf:
[general]
context=default
MusicOnHold=default
port=5060
bindaddr=0.0.0.0
srvlookup=no;yes
language=en
dtmfmode=rfc2833

maxexpiry=600
defaultexpiry=120

[502]
type=friend
username=502
secret=pass
host=dynamic
mailbox=502 at rm
callerid= "Operator" <502>
context=rm
dtmfmode=rfc2833
accountcode=
setvar=DINTERNAL=1

In extensions.conf I have hints setup that is monitored from a 601
with the expansion module.

I also have around 7 sessions connecting to the manager API over the
network using http://www.snapanumber.com/ .

Versions:
Asterisk 1.2.13 built by root @ pbx on a x86_64 running Linux on
2006-11-13 16:44:01 UTC

root at pbx:~# cat /home/Polycom/sip.ver
1.6.7.0094 for 11402_001
1.6.7.0098 for all other platforms
bootrom is 3.2.1


Please hep.

TIA