Is it illegal to send text messages directly to the State Bureau for Letters and Calls?

Not illegal. The Bureau for Letters and Calls is the petition work organization of the people's government at or above the county level, and is the administrative organization responsible for the petition work of the people's government at the corresponding level. People's governments at all levels and departments of people's governments at or above the county level shall do a good job in letters and visits, carefully handle letters and receive visitors, listen to the opinions, suggestions and demands of the people, accept their supervision, and strive to serve the people. Therefore, it is not illegal to send text messages directly to the State Bureau for Letters and Calls.