Need more information. If the switchport is not configured with a voice vlan (Cisco environment) then both the phone and PC would send DHCP broadcasts in the default vlan. The PC could pull an IP address and access the network as usual. But the IP phone could be missing info it needs from the DHCP server like option 150 which tells the phone the IP address of the TFTP server.....the PBX could be also be offline.... Lots of possible scenarios here.
