Hello Pierre,
welcome to the Polycom Community. Are you using the correct cable?
The community's VoIP FAQ contains this post here:
Aug 01, 2013 Question: What mobile phone cable can I use?
Resolution: Please check => here <=
Please ensure to provide some feedback if this reply has helped you so other users can profit from your experience.
Best Regards
Steffen Baier
Polycom Global Services