Fortinet Fortigate 110C failover problem

[esteve]

Hello; I've been working in a new company since two weeks ago and my boss told me that I need to be in charge of the networking part as well (the company is small). But the last technician didn't left any network topology nor any other network related info. I've been investigating and we have a redundant system with: 1 Fiber 100/30 MB for computers/internet 1 ADSL 20/2MB for computers/internet (backup) 1 Metrolan Fiber 5/5MB for VoIP telephony We have a Fortinet Fortigate 110C Firewall which has 2 WAN interfaces: WAN1 - Receives Fiber WAN2 - Receives ADSL Port8 - Receives Metrolan Fiber If I disconnect either WAN1 or WAN2 internet is still present, so it's working correctly. The problem is about the VoIP connections. If I disconnect Port8 the phones stop working for calling outside the company. I don't know how to configure the firewall to give the phones the current working WAN interface as a backup to use in case Metrolan Fiber goes down. In a nutshell: If Metrolan Fiber goes down it should change to WAN1 or WAN2, depending which one is in use at that moment. I hope I made myself understand, because is a configuration I never had to use at work. If you need any further information about the current configuration I will give you, because lots of things are configured here and I don't know what most of them are for. Thank you for help!