Support for the Sangoma may be making its way into pfSense before too long, there has been a bit of buzz on the topic over the last couple days.
I know at least one dev has a Sangoma card (for FreeSWITCH) and there is interest in supporting these. Someone else mentioned having a couple on order but it may be 3+ months before anything is done with them.
The FreeBSD drivers can be found here: http://www.freshports.org/misc/wanpipe/
Since it's already a FreeBSD port, it may not be that difficult to pull in the necessary bits. You'd just need to compile the KLD on a system that was of the same "vintage" (FreeBSD release) as the pfSense version you're running, copy it over, and have it load on boot.
The downside of the Sangoma cards is cost, at $425-500 or so a piece, they have quite a price tag, but there are definitely cases where it is well worth the cost.