VoIP fails to register on Gateway failure

[Brano]

We have dual gateway system. 1st gateway is preferred, 2nd gateway is used only if the 1st one fails.

When the 1st fails and 2nd gateway takes over everything works except internal VoIP ATA is showing "Trying to register" to internat side SIP provider and won't register.
We have to manually flush state table and restart the ATA, then it'll register.

We have kill states on gateway failure enabled as well as dynamic state reset is enabled.

Any pointers to fix the issue appreciated.