There are several scripts available, which transform the info of the freePBX contact manager into a file, the phone can understand.
I use a script, which I adapted for Digium/Sangoma D-phones. I prefer to use the Asterisk DPMA with the Digium phone config module…yet, it only has a limited phonebook with no freePBX integration (voicemail, cdr)…
Now I can use the freePBX contact manager and my Digium phones use it too
Similar solutions should be available for Grandstream…