I just replaced my home analog proprietary system with FreePBX plus Grandstream GXP 2130 phones. I’ve been happy. I started looking at Endpoint Manager but realized I didn’t need it. Grandstream provides an XML generator tool that works well enough for provisioning. It takes a little manual editing of a config file and setting up a directory on the web server. But you already have a web server with FreePBX. You need to run the tool on linux.
You can get the 2130s for as little as $65 a piece in multi-packs on Amazon.