To be honest I nothing springs out as blocking inbound registrations. In fact your firewall could be described as dangerously sparse. I wonder about all the static NATs though, whether these might be conflicting. I might be inclined to strip back all the NAT entries referring to your PBX except for the masquerade line.
However first let's talk about the phone system. You've said that internal phones register OK. What sort of PSTN connection do you have, and are you able to make inbound and outbound external calls? I suggest stripping down to the minimal NAT and firewall configuration to get this working in the first instance.
Assuming your PSTN connection is a SIP trunk, most ITSPs don't want a lot of NAT stuff at your end, they want your traffic to go though normal outbound NAT and their traffic comes into you as if it was replies to your connections. This applies to both SIP and RTP.
You haven't said what parameters you program into your remote phone to tell it to register with your PBX, or what protocols it uses.
Phones running in separate 172.XX.XX.XX/24 network and others are running in 192.168.0.0/24 network
my phone line is old copper SLT line its not a sip connection
i have a public ip and iam running the sip server inside the ipbx box, so that any ip phone can connect to my server as a extension.
my problem is from outside ip phone can register into my ipbx server inside successfully and from that extension ip phone i can send messages to the ip extension inside the ip extensions behind mikrotik..only issues is i cannot call from outside extensions to inside...
after digging deep into mikrotik the problem is mikrotik have SIP ALG enables by default and there it has issues and my ip phones are nat aware and mikrotik might be messing up again due to SIP ALG..
Again i ll check the IPBX config from top to bottom and if there is no issue i might be have to disable mikrotik SIP ALG and check it is working or not..
Will update once i get a solution :) the issue is with mikrotik not IPBX box up to my knowledge..
Thank u for ur time:)