assigning yealink T27G to existing extension not working

[woo60420]

We run Ver. 16 of 3cx. on windows 2016 server.The phone is a yealink T27G with correct firmware. The phone shows up in Phones to assign but when it takes you into the extension settings it will not allow you so say OK to actually add the phone to the extension. It just does nothing and the only option is to hit Cancel. If i choose another tab and then click OK it takes me right back to the phone provisioning tab but will not accept OK from there to move forward? Anyone have any ideas as to what might be going on?

 

[woo60420]

I've just found that if i delete the old phone out of the list then it will allow me to say OK and add the new phone but why would that be?