That’s interesting… since it doesn’t match my usage (i.e. it seems to be doing what /I/ need of it in my case). Can (!) you elaborate or point to some resources/links about d&u being not well supported, please 
?
EDIT:
so I googled, and found this; and this.
But I run in chan_sip and probably will until I have to migrate to FreePBX with Asterisk >12 (where pjsip will be default and “is the answer to my problems”)… I sincerely hope FreePBX team can make up a path of migration for users like me 
meaning FROM D&U to Extensions mode even semi-automatically.