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I am using T4xS phones with Firmware 66.81.0.110, the phones often loose registration with my server where the phone syslog shows "NET <6+info > [255] Cannot connect socket node / select timeout (0 ms)" after the gray out period ends the phones come back to life however the active call drops, the blf keys gray out.

Very annoying to our users, please any help would be appreciated.

Would you please also apply the config.bin file of the phone, so we can check the setting of the phone.
You can export it via setting->configuration->Export or Import Configuration.

And how many phones in you side now?

Please set the Server Expires to default parameter as 3600.
It's under web interface->account->register->SIP server.

And try to provision one parameter below.
sip.sub_refresh_random = 1