SIP problems


I have a FreeBSD router installed with Asterisk (from the ports),
this system is using by 180 SIP accounts but i get the message from “sip set debug” on many SIP phones :

<— Transmitting (NAT) to —>
SIP/2.0 481 No subscription

I can’t find any forum post about this problem, does anyone know what is wrong with it?

What was the request that is being rejected?