I've had some luck with a 6net modem that communicates via serial to the PLC in 3rd world countries. We dialed out with an analog line direct to the machine and could get connection. HOWEVER, the connection would drop regularly, so we eventually hooked the modem to a laptop and did TeamViewer so if the connection dropped it wouldn't screw up a project download.
Now, we would just use an e WON with a 3G modem in it. If you have a lot of locations, a family data plan is probably cheaper than a bunch of land lines dedicated to machines, just pop in the SIM cards and you're good to go.