SIP phone can't fallback to cucm due to fortigate

Hi Experts,

Can you please Helps me solve this Issue.

I have a fallback issue with my sip due to fotigate firewall. 

cisco sip phones goes in srst mode when wan connection is down. I have to manually reboot the phone to  get it register back to CUCM.

The when removing the fortigate ( isolating) from the architecture, phone in srst mode fallback to CUCM automatically.

when capturing trafic on incoming ports and outgoing port i can see that the sip trafic send by the phone  (received on the incomming port ) is not arriving on the outgoing port.

I have allowed "all" as service between both interfaces but sip still can get register back automatically.

Is there any thinght else that can bloc the sip traffic ?

Thanks in advance for your help

regards

Zanga