It´s recommended to use one, but you don´t have to. Leave it blank and you´re done.
Ok, how do I have to configure the Line? If I dial the IP-Adress, the trio is dialing it like a "normal" telephonenumber and so it is not working.
someone can help me with this or knows how to configure the line correctly.
well, everything is explained here:
Download the ZIP-file from this thread https://community.polycom.com/t5/VoIP-SIP-Phones/Join-a-go-to-meeting-with-Polycom-Trio/m-p/104448#M... (H323_Line2.zip 1 KB)
and import it to your Trio. After this you´ll see a button on the Trio named "H.323", tap on it, enter the desired IP and the Trio should call that IP endpoint behind it. If you want to dial only one IP automatically, edit the cfg-file with the autohook stuff from Steffen few posts ago.