I'm doing this hack job where I have routed a cell phone's GPRS modem over to a local PTY. The proprietary debug software actually routes the modem over TCP, I then use socat to route it to a PTY (planning on using PPPD, which needs a serial terminal to connect to AFAIK).
I plan on establishing a PPP connection over that PTY (maybe PPPD, or any other solutions?)
However, even if I establish this PPP connection, I have no idea how to make it effective over the OS (so it doesn't use the WIFI or any other configured options such as Ethernet). This is the part I really have no idea how to setup. I took a look at possibly setting something up in System Preferences → Network but the remotely closest options are Bluetooth DUN, PPPoE, USB modem, but I don't know what I am doing at this point. I'll even have to learn to make this successful PPP connection. We will see.
OS X Mavericks
T=Mobile Sidekick II phone with Enfora Enabler-II G modem