For our small business we'd like to run on 2 connections using PFsense. We were just wondering about a few things…

So we currently have only 1 ISP with a VPN connection to our "HQ". We need the VPN for security reasons and to create a LAN network over the internet. This is also the way we connect to our VoIP. This is not accessible from external IP's.

So!
We live in a country where even the business internet has flaws and we lose our connection and calls several times a month. So we want to  have a backup connection to take over when the primary connection fails.
We know this is very possible with PFsense and also to maintain the VPN connection.

However, we are wondering if we have a secondary connection and the primary fails. Will we still lose the calls when the backup takes over? Or is it possible to configure PFsense in such a way that the server won't see the difference and the calls will continue without any hassle when the primary line fails?

I hope I was clear enough on the matter and I'm willing to give more details if needed.
We are also willing to make a few changes in our infrastructure, if needed!

Kind regards,
Remy van Tour.