Failover ISP for VoIP

FreePBX 2.10

I have a client that wants Internet connection failover capability for a VoIP provider. Both of the Internet connections are static IP addresses. No DHCP is involved. I am able to provide a router that will automatically provide the failover capability. I am able to determine my actual external IP address without any issues. My plan is to change externip ip as appropriate when one of the Internet connections is down. My question is how to make the change so that asterisk / freepbx accept it and reload as appropriate.

As I understand from documentation that I have read (see below from, externip can only reside in sip.conf which changes must be done in the web interface and not sip_custom.conf or sip_custom_post.conf for instance. Is this correct?

externip can only appear under the [general] section of the sip.conf
If you have SIP peers both inside and outside your NAT’ed network, you will need to use the localnet parameter so Asterisk can distinguish between them.

So I am looking for the last piece of the puzzle, how to inform FreePBX of the change that has already taken place for externip?

Use the dynamic option under SIP Settings in the Web Interface. You’ll need a hostname that resolves to the public IP.