some phones 'unreachable'

[Yealink Support]

Hi dwoods,

Do they have the same firmware of the same model?
Can you register other accounts in these two phones?
1. Do you replace the network cable and test again?
2. replace a switch
3. Clear the server cache
4. Find a well port and a computer which can connect to this port and network. Then the issued phone connects this port by internet port and computer connects to phone by PC port. Please check wether computer can obtain network now.
If yes, it may be software problem; if not, it should be hardware problem.
Please tell me the model and more infos.