External interface for USSD
we already have SMPP for SMS, but don't have similar functionality for USSD, i.e. a way in which external applications can exchange USSD with MSs.
There are some provisions for USSD in SMPP, but I think they don't really appreciate the session-oriented nature of USSD.
In either case, I'm not aware of any standard to hand USSD to external applications. Of course there's MAP, but nobody wants to implement that in an external application...