I have been having the same issue with the T29g phones, purchased November and December of 2018. might unplug a working phone then plug it back in and all you get is black screen and solid red light top right. I have asked if there was a way to fix this, so far no answer from Yealink tech support, My tech support for the pbx just gives me a rma and I get a new phone. I have had 5 go bad out of 30 so far and expect more.
I hope they have the new ones fixed for this. PS all the phones have been hooked up to PoE switches.