We are looking at using a arduino setup as a modbus slave, but would like to poll it using the sm5100b cellular shield. Ive been searching around but cant seem to find anyone that has done this. Most modem manufacturers support this and have modes built in to their devices to accomplish this. It is often called tcp sockets, modbus rtu over tcp/ip, encapsulated modbus, etc. digi, moxa, sierra wireless, all make modems that support this operation.