Group Series joining Skype meeting issue

Hi,

The Group Series 310 is calling another Skype for Business user instead of joining the scheduled meeting. For ex: instead of joining the meeting id 302938, the GS calls the user with the extension 302.

The GS is correctly registered to the Skype for Business, Directory and Calendar servers. SIP is configured as first option for audio and video. We also created a specific DialPlan on Skype without any normalization rules.
Do you know where this problem can come from?

The software release is 6.1.2.1 and version 380105.