My priorities

[wibble]

Is it possible to have a VoIP app that allows the Pinephone to act as a SIP/IAX2/other VoIP protocol trunk via WiFi? Using the Pinephone as a second phone, it's somewhat difficult to justify the monthly cost, since I don't really need it. But if I could make and receive calls through an Asterisk PBX over WiFi when I'm home like a landline, it'd make the phone a lot more useful.

Basically: IP phone -> Asterisk PBX/other VoIP PBX -> Wifi -> Pinephone > Cellular Network

I know the software isn't there yet for this task. On Android phones I thought there is some kind of restriction that prevents this.

You should be able to do that by running Asterisk/otherPBX on the PinePhone, assuming capabilities haven't been removed since people were doing that ~10 years ago. I'm not sure if you can build wake on wlan into the process so you can let the Pinephone sleep - back then the wifi interface didn't support it.
http://wiki.openmoko.org/wiki/Asterisk