If it were possible to install a BSD on a smart phone, the best bet is
NetBSD, but I doubt that's possible. NetBSD works with more hardware architectures. The architecture of that phone has to be supported by that operative system. If something for that is only available in Linux, then use that. Something like this would be for browsing the Internet on a smart phone through your home wifi.
There's Pinephone,
https://www.pine64.org/pinephone/, which is a physical smart phone that works with opensource software. It comes with its own version of Linux.
If you want a phone system, that goes on your computer, that allows other softphones and hardphones to work off of, try
Freeswitch or
Asterisk as a PBX (Private Branch Exchange). Using
TrueNAS or a custom GUI OS of Freeswitch or Asterisk would be easier than installing a PBX on FreeBSD. These go on the computer and not on the individual phones that use the PBX system. I never heard of using a smart phone with open source software as a soft phone for use with Asterisk or FreeSwitch. There's softphone software, though this is for a computer. If it were possible, it would be using the phone over wifi, and not through a SIM card.
I use an old smart phone over Wifi for browsing the Internet, but it retains the original Android operating system that came with it. It always has an indication that it needs the SIM card. I haven't even checked if the Android/Google store is accessible on it. I doubt it is, and I wouldn't try it without a working SIM card, because it's probably not intended to be. That's something you would want to read about online before even trying. There's nothing for SIMs in terms of opensource that would only work on Wifi or a bring your own hardware phone IP phone service.
As a similar task, for me, installing a BSD or Linux firewall on a gateway is something I can't wrap my head around achieving. There's operating systems made for that, but there's no place to put the USB for installing it, maybe it's done over the network. Using Raspberry Pi or a BugleBoard which are supported by many open source operating systems is something I haven't tried, but I could remotely see myself trying that. Operating systems are meant to work on many of those hardware architectures, as they're listed on their download pages. I can't wrap my head around installing FreeBSD on an old smart phone, and it won't work if the phone's hardware architecture isn't supported by the particular OS. I've been here a while, and can't do many of these tasks which are possible.
