Running Raspbx on Pi zero to see if it can handle it, seems to work fine, originally setup on PI2B and then card put into Zero, few oddities originally, but sorted those out.
Have it connected to Sipgate, friends Raspbx setup similar to mine, he has PI2B, and to BT line using Grandstream HT-503 in my work shop, plus Linksys unit to BT line at home, plus extensions.
Calls to workshop also ring home and vice versa.
Works very well, so when abroad or alternate location can use softphone, or hard phone and make/get calls from UK.
Using with a USB to ethernet with 3 port hub bough from ebay, had a usb C type plug , so cut and splice cable to fit Zero - recognised immediately. item number 162049671280 USB to LAN Adapter USB-C RJ45 Gigabit Ethernet Network 3 USB 3.0 Port Hub