There are subtle issues with FXO devices and pjsip. Simplified, on an incoming call the FXO device puts the caller ID in the From header, where pjsip normally expects to find a user ID to match the trunk.
One solution is to use a chan_sip trunk instead.
Depending on your Asterisk and FreePBX module versions, you may be able to fix the pjsip match priority from the GUI, or you may need to edit the config file manually. For the gory details, read these threads: Can’t get FreePBX to fully work with ht503 and No incoming calls .