welcome to the Polycom Community.
Is this an audible message or is this displayed on the screen of the SSIP6000?
As you only provided little Information I can only guess but the community's VoIP FAQ contains this post here:
Oct 7, 2011 Question: Phone unable to Dial a number when Off Hook or on 2nd Call in a Conference or Digitmap issues
Resolution: Please check => here <=
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Do you have other Polycom phones that work when dialing the same extention numbers?
Looking at the 'missed call' list does it show users like 'joe' or 'sip:email@example.com' or does it show '1234' or 'sip:firstname.lastname@example.org'?
Many PBX systems set up extension numbers as aliases to string style extentions. If the dial plan is not set up to handle these differences then it may not recognize the extention. and send a voice message back.
When you miss a call, the PBX will send the correct contact info to the phone. This might be why the 'missed call' list works, but direct dial does not.