Welcome to the Community and thanks for your post!
Sorry to see you're having problems on two of your handsets. Which model of handsets do you have? If you have the user guide handy you can get the number for product support on there. Contacting them would be your best bet.
If you can't find the user guide post back and let me know the model of handset and I'll find it for you online.
I have managed to sort out the problem by removing the batteries and leaving the handset for a short time before replacing the batteries.
The handsets re sinc. and they all now ring.
The handbook tells you how to shut off the ring and put it back on but does do help if this does not work.