SIP Authentication methods available?

I have an AS-5300 used by several SIP phones (mostly through Asterisk)
to dial from VoIP-land to POTS-land. My current "authentication"
method is rather draconian, poor and inflexible: I just played with
the routing tables to make sure that the only phones that can have IP
connectivity with the voice gateway are those in certain subnets.

Now, I would like to move up to something that p ..
Telecom, Cabling and VoIP continues...
Tags: telecom cabling fiber optic infrastructure LAN