HP 4120 Lync Phone Network issues

Question

Hello Lync community!
We have a 2013 deployment running HP 4120 desk phones.  I'd like to see if anyone else is having similar problems to ours.  First things first: HP Lync Edition phone is manufactured by SNOM.  While support/warranty comes from HP the packaging
materials on repaired units say SNOM.  With this fact out of the way let me tell you what we are seeing.  On initial roll-out the failure rate "out of the box" was about 5%.  The downstream port would not function - and we needed it
to connect the workstations in the offices.  The lights on the port would flash, but once you flipped the phone over and made a few calls the port would start acting up.  On some phones the port would just go dead, on others it would come in and
out - causing endless frustration.  You can typically fix the condition temporarily by unplugging and plugging back in the cable in the downstream/PC port.  Needles to say these were sent back to HP for repair.  By the end of 1st year about
another 5% of the phones had to be replaced due to the same issues.  Initially we thought that some type of electrical issue on the phone side was at play - perhaps a poorly soldered connection, a batch of bad/loose network jack - something that has to
be addressed on the manufacturing side of things.  Since then also learned that phones themselves can enter a state in which they remain powered on but claim no network availability.  The phones are all powered via PoE - so they are connected, but
somehow they claim the lack of access to a network at an Ethernet/Level 2 level.  A phone reboot fixes the problem.  This makes us thing that the issue is not in wiring, but in some low level firmware on the phone - not the WinCE 6.0 the phone is
running - but maybe something else.  Phones run the latest UC software available from Microsoft.  We have some Polycom VVX phones in identical setup - those have no issues with the network. 
Anyone seen this?  Share your experience - let us know what you've done.  We have no issues with phone provisioning, dropped calls or unexpected sign-outs.  Lync farm running latest CU as well.

Answer 1

After writing the post above I did another experiment with the phone. 
Took it apart to see if I can notice something obvious on the PCB - noting there - all is firmly attached and well soldered.  Used a magnifying glass with high magnification to verify that there are no cracks in the solder where RJ-45 jack attaches
the to the PCB.  Put it back together - plugged in the network cable.  Phone boots, asks me to sign in - as I start the process and begin handling the phone I get disconnected from the network.
The error message that the phone shows:
"The Network cable was not detected. Please check that your cable is connected to the network port and that you have a network connection."
In this state the lights on the network port are off - they are not blinking.  So the PoE is delivered but the interface is down.  Looking at the port status on the switch shows that  PoE is "delivering" but the link status is "down". 
All you have to do is firmly press the network cable back into the phone and it starts working.  Go figure what's at play.