I'm configuring a Netvanta 3140 for a customer. It is functioning as an SBC between their Asterisks IP PBX and a SIP service provider. They want to enable Digest Authentication in both directions between the Adtran and their PBX. I can authenticate outbound calls to the PBX by configuring the 'Authentication Username XXX Password XXX' command under the Trunk configuration, but I haven't found a way to enable basic MD5 authentication for SIP calls to the Adtran from the Asterisks. Is this possible? Thoughts?