Service unavailable?

OS Version:
Linux argosrecords.com 2.6.17.11-1.1.x86.i686.cmov #1 Tue Sep 5 23:54:38 EDT 2006 i686 i686 i386 GNU/Linux

Asterisk Build:
Asterisk 1.4.0
Asterisk GUI-version Revision: 290 $

i have tried changing the server name i used sip.broadvoice.com and also using just the ip have checked all my network configuration and all seems to be fine its not an issue with the router or firewall becuase i had the system up and running on Axxon pbx prior to this install

and i still and getting the error saying service unavailable

any help would be great

eb 13 11:12:06] NOTICE[2594] chan_sip.c: – Registration for ‘6625804020@147.135.12.128’ timed out, trying again (Attempt #254)
[Feb 13 11:12:26] NOTICE[2594] chan_sip.c: – Registration for ‘6625804020@147.135.12.128’ timed out, trying again (Attempt #255)
[Feb 13 11:12:46] NOTICE[2594] chan_sip.c: – Registration for ‘6625804020@147.135.12.128’ timed out, trying again (Attempt #256)
[Feb 13 11:13:06] NOTICE[2594] chan_sip.c: – Registration for ‘6625804020@147.135.12.128’ timed out, trying again (Attempt #257)
[Feb 13 11:13:26] NOTICE[2594] chan_sip.c: – Registration for ‘6625804020@147.135.12.128’ timed out, trying again (Attempt #258)
[Feb 13 11:13:46] NOTICE[2594] chan_sip.c: – Registration for ‘6625804020@147.135.12.128’ timed out, trying again (Attempt #259)
[Feb 13 11:14:06] NOTICE[2594] chan_sip.c: – Registration for ‘6625804020@147.135.12.128’ timed out, trying again (Attempt #260)

if you are behind NAT, make sure that externip= and localnet= are set. also make sure it’s set nat=yes as that can sometimes cause funny problems. also look @ a sip debug and see if anything pops out…