Hi,
To my knowledge the usage of USSD signalling requires a gateway - one gateway per mobile operator/network.
The gateway's task is relaying the USSD-codes from the mobile phone to the receiving remote server.
I'm not sure If all mobile operators in the world provide such service.
To my knowledge the mobile operators in Norway use USSD inbound only; signalling to ValueAddedServices like (prepay, voicemail).
I have not heard that any USSD-gateways are available for external customers, and if it exists, I'm sure there is a cost of using.
However the M2M-marked is growing, and I think USSD is a good candidate for fast transfer of small amounts of data.