It has now come to the time where 3PIP is about to die. This is the message in M365 to look for.
So what are your options?
Option 1. SIP Gateway
Microsoft has a SIP Gateway that can be used. This is some job and firewall that has to be done before you can start using the service. Read about it here: Configure SIP Gateway - Microsoft
When this is done you can use the phones for more times.
Option 2. Buy Teams phones
Another option and what some of my customer are thinking about, is to get new Teams phones. They are already planning for this option. And yes it cost money, but it still cost money to have old phones running. And since some of them are EOL (End of Life) and EOS (End Of Support) well. If you are on Teams and plan to use phones. Well do it correct and get Teams phones.
List of certified Teams Phones can be found here.