Below changes were added.

• Added TCP 5060 for SIP(As sometimes this can be TCP/UDP) for all WANS
• RTP port range 6200 – 6214 added for Inbound for all WANS
• SIP domains allowed for Inbound for all WANS

SIP ALG turn off – Need to run below commands if it’s required. Best to test the phones after above changes.

en the Fortigate CLI from the dashboard and enter the following commands:

• config system settings
• set sip-helper disable
• set sip-nat-trace disable
• reboot the device

Re-open the CLI and enter the following commands:

• config system session-helper
• show    (locate the SIP entry, usually 12, but can vary)
• delete 12    (or the number that you identified from the previous command)

Disable RTP processing as follows:

• config voip profile
• edit default
• config sip
• set rtp disable

config system settings
set default-voip-alg-mode kernel-helper-based
end

Important is that you need to configure it on all the VDOM`s
 
A reboot is not necessary, Clearing the sessions worked for us:

diagnose sys session filter
diagnose sys session filter dport 5060
diagnose sys session clear
diagnose sys session filter dport 2000
diagnose sys session clear

Great diagnosis guide as well here