The loss of BToE functionality with the 3pip phones is unacceptable. Who at Microsoft thought it was a good idea to take away users ability to initiate phone calls with the desktop app but used the desk phone for the audio functions is not seeing the experience from the users point of view.
It is great to say "these phones will not work as you are used to when we move your to teams, so you should buy phones that do," when phones that will be supported are not out yet. We had an office full of CCX600 phones that needed to be replaced because of 3DES depreciation and the only viable replacements were 3pip phones.
You need to develop a bridge to keep the basic calling function integration. No one cares about the "Teams functionality" on a desk phone, just making and receiving phone calls with the convenience or right clicking on a contact and selecting call.