Note this is only doable with version 2.0 and newer, it is not possible to do this on any earlier release.
To configure a 3G modem on pfSense, plug in a supported modem, and log into the web interface to begin configuration.
Browse to Interfaces > assign, and click the PPPs tab. Click + on that screen.
In the Link Type drop down, select PPP.
In the Link interface(s) box, the list will be populated with serial ports on the system that respond as a modem (return OK in response to an AT command). Select the port for your modem.
Optionally fill in a Description, which will be used to reference this PPP configuration in other parts of the web interface.
Under Service Provider, select your Country. The Provider list for that country will appear, where you select the provider of your card. Then in the Plan drop down, select your plan. This should adequately fill in all the PPP details needed for the connection. Click Save.
At the PPPs screen, you will see your newly created PPP interface.
Next the PPP interface must be created. Click the Interface assignments tab, and click the + to add a new interface. Select your PPP interface, click Save, then Apply changes.
Now browse to Interfaces > OPT1 (or the interface you assigned the PPP interface as). Check the Enable Interface box, rename it if desired, and click Save. Do not change anything else on the page. Then click Apply changes.
Browse to Status > Interfaces to check the status on your newly created and assigned PPP interface. If it does not show connected, check the logs under Status > System logs, PPP tab to see why its connection is failing.