Cellular routers and gateways can be accessed via TCP by Telnet or SSH. You don't have to connect these devices to a PC, they just need to be connected to your network and configured appropriately to receive messaging commands.
Good examples of cellular routers/gateways are:
Multitech's MTR-LNA7, MTR-LVW2 and MTR-LAT1
Airlink's GX450, RV50, RV55 and LX40
Cisco Series 800, 900
For clientele in the UK and EU regions
ConiuGo GmbH 700600150S
ConiuGo GmbH 700600160S
ConiuGo GmbH 700600170S
For information on how to configure TCP accessible cellular hardware, have a look at the Creating a GSM-AT-IP Carrier steps.