I found an old T-Com Speedport W 500V and would like to use it to connect an phone of similar age to VoIP via Wifi.
OpenWRT runs fine when I do a custom compile but I have no idea how to use the router's analogue telephony port. The Wiki page only lists the DSL modem as unsupported so the PBX (I've come to understand that's the official name for those ports) should work, right? Asterisk seems to be the way to go but that's way to large for the 4MB flash of the Speedport. Are there any alternatives?
4 MB flash and even more the 16 MB RAM disqualify this particular device from the get go, but there are no drivers for Broadcom xDSL, FXS (analogue phone ports) or DECT (not present on this particular device) hardware either.
The only OpenWrt compatible SOCs with xDSL modem and FXS support (but no DECT either) would be the various generations of the lantiq lineup, but to keep your sanity only the 'VR9' VRX268/ VRX288 would be worth looking at (you can find several ISP branded devices for 5-10 EUR on the used market).
Even thinking about running a SIP pbx (asterisk, as you'd need to use it for FXS support on lantiq via the asterisk plugin chan_lantiq) on a router requires a lot more hardware ressources than the Speedport W 500V can deliver, 16 MB flash (without removing hardware support for modem or WLAN, an asterisk+chan_lantiq firmware image is around 10-12 MB in size) and 64 MB RAM are the bare minimum for this to work (again, on lantiq, broadcom isn't an option to begin with).
Thanks for the details, saved a lot of time for me. I just thought I could turn the router into something useful and was delighted to find it supported by OpenWRT. There's still Bitswitcher but it seems to be abandoned.