Digium D phones work best, if you use the Digium module in freePBX and if you want to finetune the phones, read this:
https://wiki.asterisk.org/wiki/display/DIGIUM/Advanced+DPMA+Configuration
I use several D65 in a production environment and they perfectly work, even with the newest firmware! Visual voice mail is a Digium app, don’t use Sangomas phone apps!