I'm trying to implement the following solution: and elastix PBX behind a dual WAN firewall. When one WAN link breaks, the firewall automatically switches over to the other WAN link. The problem is this: my SIP trunk provider does not work with DNS---IP only. I understand that the sip_general_custom.conf file has the line externip=220.127.116.11 and that this is vital to the communication with the SIP trunk provider. My question is this: is there any way to set up Dynamic DNS on the PBX itself so that when the WAN link switches from the primary to the secondary (on the firewall), the sip_general_custom.conf file is updated accordingly thereby allowing continued communicatons with the SIP trunk provider?