normally, the PC will need only 2 cable pairs from the ethernet bundle. (normal ethernet connection)
Now, If you are using Power over Ethernet for phones like I do , then all 4 cable pairs will need to be connected end-to-end (switch to phone ) If at least 1 of the cable is not terminated or damaged, the phone goes down or gives you Ethernet disconnected error
this can explain why PCs are working fine, and phones are not working.
it has always been cabling issue for me till now.
also, from experience, any new data point which is being laid, I use an IP Phone to test it.
happened to me, the guy tested the a new data point with a laptop, but my phone doesnt light up, and he says he did his homework well. !