Phone only ringing after reboot- quickly stops working

[AE]

I am having an unusual problem this past week.
We have an W52P with 2 handsets. The phone will always make outgoing calls but it often does not ring for incoming calls. I know the SIP provider/ DID number works because our DID forwards to a cell phone as well as the W52P and the cell phone always rings.
I haven't changed any setting on the W52P unless a firmware update has happened?

If I reboot the W52P the handsets will ring when I test it but after some time they stop receiving calls.

Here are the settings I'm using, any ideas why they would stop picking-up incoming calls?

Thanks very much in advance!