Polycom Group Series systems running firmware 5.1.1 do not have ability to use the directory service.
We are engaged with Polycom to resolve this issue, as of this time we do not know if Polycom will resolve this with a hotfix or a totally new software version.
Until this has been resolved Videxio support recommends using Group Series firmware 5.1.0.
Update 2016-06-27: Polycom Group Series firmware 5.1.2 has been released and we are currently awaiting test results.
Update 2016-06-29: It has been confirmed that this issue is still present in Polycom Group Series firmware 5.1.2.