SIP Traffic VIP Natting not working

[technick]

Hi all

We are using SIP Trucking at our facility we had it working great until recently when we swapped ISPs. 

We have SIP Helpers on on the Fortinet and never had a issue till now... 

Here is our setup 

Our SIP goes over the internet and fails over when a WAN cannot be reached. We are having issues with our WAN2 invites from our phone system flowing over to WAN1. I notice this ONLY happens when the session is expired and the phone system initiates it out.

When I call in the session is created from externally and keeps the second SIP Session going for a bit. 

Is there any settings i should look at or change?

SIP Trunk 1. WAN1

SIP Trunk 2 WAN2

[gfleming]

Might need some more info here. Are you saying you always had two ISPs but you just changed one ISP and you have these issues? You did not make any other config changes (SD-WAN, etc) on the FGT?

Also what do you mean WAN2 invites are "flowing over" to WAN1?

And by session expired you mean it's expried on the FGT in terms of state or its expired on the phone system?

Do you have two separate SIP trunks configured to your provider over each of your ISP links? Why not one trunk that automatically swaps over as needed based on your ISP being up or down or even out of SLA (if using SD-WAN)?