I've recently came across a very cheap internal dial-up modem on Mercado Livre. It cost me the equivalent of about five American Dollars. Upon further research (before buying it, obviously), I've found little to no information on the Ambient MD3200 chipset, but I found out it could be used as a X100P/X101P card on Asterisk, thus providing my server with an FXO port.

Albeit POTS is on it's sunset as an ubiquitous technology, this was cheap enough to convince me to play a bit with that technology, since I still got a log of POTS or POTS-compatible gear around. But information on DAHDI configuration is a bit harder to come by, those days. It can be found, fragmented through dozens of forum posts, but there's no tutorial explaining the whole process.

Initially, I've decided to use Freewitch, but mod_freetdm, albeit still somewhat maintained, lives outside of the source tree and the documentation is not exactly good. I wasn't able to compile and get it running on Freeswitch. So I decided on Asterisk.

Asterisk has excellent support on DAHDI. Zapata Telephony Project, or Zaptel, was written by Jim Dixon a long time ago, incubated by the Asterisk community. Now, Sangoma keeps maintaining it, even though both digital and analogue trunk lines are becoming nearly extinct animals, with the onset of SIP technology, now prevalent.

My first idea was to do a PCI passthrough of that card to a virtual machine. That would provide optimal isolation and keep both the kernel module and Asterisk fairly isolated from the host. Unfortunately, this dumb board was sharing it's IRQ with SMBus. Some PCI hardware can deal with this, this card couldn't. So I decided on installing the DAHDI modules directly onto the host. Not ideal, but it is what it is.

Then I decided to run Asterisk inside a LXC container, passing through the device descriptors to the container. Despite that, we do not need to make it privileged.

This as kernel module: https://github.com/asterisk/dahdi-linux.

It should be installed at the host.

I've followed those instructions, for the largest part: https://docs.rockylinux.org/guides/communications/asterisk_installation/#asterisk-configuration