In my SIP network there is a Cisco PGW 9.6(1) used as PSTN gateway. PGW call agent controls many Cisco AS5400 via MGCP. We use Linksys SPA2102 to connect fax and modem devices.
Both PGW and AS5400 are configured to support FAX passthrough upspeeding via NSE event. Also SPA2102 devices are configured to support FAX passthrough NSE.
After many tests and debugs I see that SPA2102 doesn't recognize NSE event sent from AS5400.
From debug and wireshark traces you can see that AS5400 sends NSE 192 event correctly (see attachments).
So I've try to compare this scenario with a working one. The only difference is in the Payload content of NSE messages. With AS5400 the Payload is c0000000. I think tath this is an empty payload while with different gateway or ATA the payload is c096c4e. In this last case SPA detects correctly NSE message.