How to load-balance sip properly?

[echo]

Hello!

I have a problem with sip registration. Our customer has FortiGate 60D and two internet connections. If there is a failover event then all the voip phones switch over to the backup internet connection and continue working. The problem is if the main connection reestablishes that the registration traffic (sip / udp 5060) stays with the backup internet connection but the data goes back to the primary, and the phones don't work properly.

As for background there is very simple rule for all cases and with all routers at least in this country: turn off sip alg wherever this may activate and it works. But with this special case where there are two internet connections AND both are actively in use, that is, they have equal distance in the routing table, then this problem appears. If the distance would be different, that is, one of the internet connections stays idle and is not active, then the switchover works as it should (there are other clients who have such configuration).

Maybe this situation requires some very specific sip alg configuration but it means I have to turn sip alg back on and then get the phones just working in the first place and then in addition configure it further to get the load-balancing also working properly, which sounds like months of work. Has anybody experiences with such situation?