I have been using D65 phones and a D80 phone with freePBX and realized that the phones have additional features with Switchvox. For example, you can see on your phone with whom a colleague (info at blf button) is speaking with (if you want). All in all the D-series phones are much better products. You can search for a name, by pressing each key only once…and the phone will combine the info (2ABC) to find a name or a number. Furthermore the audio quality is great (ulaw, g722, opus16) and the phones are 100% stable, not so the S-series.
So, there is a reason why Switchvox is more expensive…and that’s ok 