Yes I've used GT-540-3GWA to get Modbus data over the cellular network. I connected my modbus devices over the RS-485 port. I configured it with the software utility that came with it and was able to get Modbus data from a PLC and I/O.
GT-540-3GWA: $699.00 Cellular Wireless Intelligent GPRS Data Server with 6 Digital Inputs, 2 Digital Outputs and 1 Analog Input. Supports 2G Quad-band 850/900/1800/1900 MHz Frequencies and 3G 850/900/1900/2100 MHz. Provides Wireless Access to Modbus RTU devices. Supports operating temperatures from -25°C ~ +75°C (-13°F ~ +167°F) http://www.icpdas-usa.com/gt_540_3gwa.html?r=maria
