Thanks for the detailed explanation.
> When you try calling mobile phones, could you please confirm the traffic flow?
> Is this correct - Phone -> Local SIP Server -> SIP provider on the Internet
> So this is Internal to Internal and then Internal to Untrust
> The signalling is failing between the SIP server and SIP Provider right?
> Trying to disable and enable the SIP ALG is a quick test to see if it helps resolve the issue
> set security alg sip disable
> delete security alg sip disable
> Please provide output of "show security flow session extensive <filters>" for the impacted flow
> This will give details on the bytes exchange and if ALG is taking effect
> Since you are seeing a flow I do not doubt the configuration
> FW is in an L3 mode, any reason why you have VLAN VoIP defined?
> A pcap is always useful when troubleshooting VoIP issues
> Would you be able to collect one for the impacted flow?
> PCAP on SRX Branch - https://kb.juniper.net/InfoCenter/index?page=content&id=kb11709