How do I route internet through local PPP connection I have on PTY terminal? 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.

Background
OS X Mavericks
T=Mobile Sidekick II phone with Enfora Enabler-II G modem
pppd automatically registers the PPP network interface upon successful registration!
From my experience performing this on OSX Mavericks 10.9.1, the /etc/ppp directory starts out empty.
Populate /etc/ppp
I had to populate it myself, and I did so from my Ubuntu installation which was already configured properly. These are the files I happened to import based on my needs:

/etc/ppp/options
/etc/ppp/pap-secrets
/etc/chatscripts (optional) - you only import this if you are using one of these chatscripts yourself.

Delete Default Route before running pppd
It might help to remove the default route before running pppd to assist in enforcing that the PPP connection becomes the default connection to the internet:
note: I found this more of an issue on my Ubuntu installation, but haven't had to this on OSX. Still here are the commands:
OSX command: sudo route delete default 
Other Linux:     sudo route del default
When you disconnect pppd and reconnect your regular internet, the route will be restored.