Has anybody ran into this issue on New Yealink T27G I program the phone it works great at my office! I take it to a job location and it asks for the last 5 digits of serial number and the phone resets back to factory.
You program the set on your company network and them plug the set into an completely different network with a completely different IP address and subnet mask...different DHCP server and all that. I believe the network configuration setting would be incorrect.
I could be wrong...as my wife is more than willing to attest to.
You might try stetting it up at your office, powering it down for a little while, and then powering back up at the office and see what happens. If it defaults then there is some kind of problem with the phone losing information after it loses power. If it stays the same then it might be that it is trying to pull information from the local phone server, or LLDP server.