@Joram, I've encountered some operators who parse most of their error codes into the pesky "*...invalid MMI...*" for display on the handset (BTW the exact notification varies from one phone manufacturer to the next). The exact spread of that 40% failure rate will give pointers to the nature of the problem. - Is it 40% of all attempts from the same handset? If so, there could be a resource problem with one of the signalling stack involved; particularly on very busy networks. - Is it failing consistently for some select handsets, translating to the 40%? If so, check the Data Coding Scheme at play. The USSD-GW vendor may need to look for ways to maintain the DCS of the incoming traffic. Either way, an on-the-wire trace (snoop/tcpdump/wireshark etc) between the communicating entities never lies - that's my mantra [?] PS : You may need to check the content of the messages at play....most equipment vendors do not take raw special characters kindly. Tony.
