OK, so we need to find out why it’s failing to register.
On the FXO page, confirm that you have
Account Active: Yes
Primary SIP Sever: 192.168.2.243:5160
(Replace 192.168.2.243 with the IP address of the PBX and 5160 with the value of Bind Port for chan_sip, if either of these have changed.)
Outbound Proxy: (leave blank)
SIP User ID: 03XXXXXXXX
Authenticate ID: 03XXXXXXXX
Authenticate Password: (match secret for the trunk)
If it still won’t register, use SIP debug to see registration attempts. If you don’t see any, they are possibly being blocked by FreePBX firewall; briefly turn it off as a test. You can use tcpdump to see if any REGISTER packets are coming in.