Could you please try to get interface list from PBX with different SNMP tool - for example, with GetIf (http://www.wtcs.org/snmp4tpc/getif.htm)? If other tools will return empty interface list, than we will know that problem is with PBX SNMP agent, but if you will be able to see interface list with other tools, that will mean the bug in NetXMS.
Best regards,
Victor
Best regards,
Victor