Note that you are likely referring to a 2nd ISP not just a 2nd IP address. If the connection to your ISP is broken, all IPs on that connection are down. In fact, if the connection to your ISP is down, the entire block/area may be down so a 2nd wire to the same ISP is pointless. Thus when most clients say "2nd IP", what they really mean is 2nd network connection.
Major Colocation Facilities will have Two to Ten physical connections to multiple providers/networks. If one goes down, they route traffic through the others. A
"Virtual IP" situation can even be set up (with some very hefty equipment costs) that even allow an IP expected to arrive through a "dead" provider to route instead through a different provider. This is not cheap, or something you'll likely be doing without an eight figure budget.
But multiple ISP/IP providers is doable.
Can be managed in several fashions:
1) pfSense is possible, but complicated. Some get lost in the challenge. Not gonna get into it, but I can say that we've had several clients over the years who had networking issues and the solution was to remove pfSense and *poof* everything works. We've also had many clients who were successful, so ... it's purely a question of whether you can handle multiple PBX connections through a pfSense system (control and audio require different ports for one call, it can confuse pfSense).
2) Dual Connection Router! There are many routers capable of "shunting" when a primary network signal drops. Drawback: Asterisk SIP connections require sip.conf to have a value for "externip" which is the public IP of the server. When the primary dies and the backup takes over, that value will be wrong and calls will fail. So adjusting that value during a "primary down" will be required.
3) Extra NIC in the server, second direct ISP connection (with 2nd IP) on the Vicidial server. This is doable, but requires special networking configuration in the OS to work properly. There will be challenges that will need to be dealt with, such as externip again.
4) Heavy Duty Router: Network professionals (often with Cisco certification) can build special routing tables for almost any situation, including double physical networks. This requires a more high-end router and is usually reserved for backbone systems (since "The Internet" is designed for auto-failover). Those routers generally run $2k-$25k, depending on capacity and features, and often require special training to configure.