Flash sms it is the same as usual sms except data coding flags. It has "class 0" indication in DCS.
For details about sms coding see: GSM 3.38 specification / 4 SMS Data Coding Scheme.
Some fragment from spec about it
When a mobile terminated message is class 0 and the MS has the capability of displaying short messages, the MS shall display the message immediately and send an acknowledgement to the SC when the message has successfully reached the MS irrespective of whether there is memory available in the SIM or ME. The message shall not be automatically stored in the SIM or ME.
So, in most cases, phone just displays this message and skips usual chain of actions.
Probably, you can't catch this message with usual API... probably you need deep hook and rooted device.
[Or may be just switch off this service by calling customer support?]