Hi all,
Since no one replied and I'm tired of waiting, we've made our own debug Nokia phone. I gave up searching for a cable and we just soldered cable to the M-BUS pins. We used old data cable for Siemens as a USB-Serial converter, they were PL2303 based. This works like a charm and is a very cheap solution. If you lack special cable for your Nokia - take this way into consideration.
To my surprise, latest Wireshark is able to decode everything I need - all USSD messages. I didn't expect this. Many thanks to those who contributed all that code to Wireshark.