The Sangoma D65 is actually a Digium D65. Digium was bought by Sangoma. Thats the reason why the D-Series phones are different.
The D-Series phones have way better audio than the S-series. The line buttons can be virtually multiplied to more than 100 (on the D65 use the grey bar underneath the line keys to scroll), I think.
The design is great!
Yet the phones work best with freePBX when you use the freePBX Digium phone module. If you need it, you can configure them using the advanced DPMA configuration mode.
I think they are actually the best freePBX phones 
Dont use the EPM or the phone apps, the D-series phones have their own apps.
You can use the freePBX contact manager, but you need a cron job and a conversion script, when you dont use EPM.
I never liked EPM & phone apps…they are supposed to work with many phones…in reality EPM works just with a few brands…it’s a never-ending construction site…on most phones you only get basic functionality…and the phone apps are just too slow to load…and are badly designed 
on the other hand, the freePBX Digium phone module is the GUI for the Asterisk DPMA, so it is a built-in Asterisk phone module…I can tell the difference…clearly
You wrote “phone design not great…”, well I do not agree…
