asterisk users - Aug 2010 - Should external hosts be able to register on Asterisk behind a firewall?

Of late I have noticed bruteforce attempts to register to a homebased
Asterisk server which is behind a router firewall and I need to know
whether the router on the firewall is the culprit here. It only stops
after the router is restarted.

The router is the HG521 from talktalk and the firewall setting is set
to talktalk and there appears to be know where to check the firewall
setup. The firewall configuration is called talktalk am not sure what
that setting means, but it has to be pretty insecure.

There is no portforwarding setup on the firewall and returning
connections should only originate from the server the Asterisk
connected to, which is just one.

I am beginning to wonder what else is trying to get through that firewall

Has anyone observed this issue?

These are 2 of the IPs I have noticed the attacks coming from.

80.193.131.93
205.234.219.232