FortiVoice stopped sending SIP registration packets

Hello, we have an ongoing problem with our Fortivoice. Without any known change in our network, FVC stopped sending a sip registration packets to our provider, the only packet that is sending is SIP OPTIONS - request. Since I have very little experience with SIP, I have tried to connect to our provider with alternative softphone app and did a packet capture. We can exclude problem on provider's side, since I was able to connect softphone and make a call. I have compared packet captures from softphone and FVC and I assume that FVC isnt sending registration packet, only an option packet, which in softphone case, was sent by provider. I have tried several solutions including good ol' reboot, firmware upgrade, disabling SSL inspection on our firewall. The provider's side is reachable, FVC is able to ping/traceroute it and provider sees incoming traffic from us. I wasnt really able to find a way to debug SIP on fortivoice and seems like noone else is having similar issue. Fortivoice is configures with default sip trunk profiles and credentials are correct. Don't know if attaching a pcap is necessary, since it contains all credentials and addressing information. regards, jk