Ad Daivd55 was mentioned, if you have BGP4 routing and dual providers this problem wouldn’t exist, because both providers are routing your IP addresses, but I’m guessing this is not the case.
I have clients with this exact configuration and you need to force all traffic out of a specific route on thre router/firewall and define SIP as the IP associated with that route on the PBX.
That I’m aware of you need to manually switch both firewall and SIP at the point it fails and there is no way to make it happen automatically.
I will further define this for someone reading it and saying of course you can…
Yes most firewalls will configure fail over and that will work, but your PBX will have the wrong SIP IP and once the call connected will almost immediately drop as well as media not communicating.
I’m not sure if Schmooze HA will change the SIP IP, that may be something to research.