Making some generic affirmations will not fix your issue. There are many messages because there are different PBX networking setup. Here are most common:
1) PBX Inetrnet ZS NAT SIP Clients — zhis is what I use, should be no setup / config however you can check conntrack and sip kernel module options. However I found that I need to do a contrack – f command every time I reboot ZS, else some SIP clients will not register.
2) SIP Client Internet ZS NAT PBX . Here u should forward ports or place PBX into a DMZ zone. ZS doesn’t have a DMZ section like a cheap router you will have to create it.