The current 3cx templates for the grandstreams has the PC Port Disabled.  This means if you have a PC plugged into the PC port on the phone it wont work.  


Luckily this is an easy fix.


Login to your 3cx Management console and goto 

Select the extn you need to enable, goto the Phone Provisioning tab and tick  Enable VLAN for PC PORT and leave 0 and 0 on