IP Office 1408 Phones Not Ringing?

[jokermia]

I have a site with (1) 1416 phone and (5) 1408 phones, Basic Edition, R8.1(69). All of the phones are set to the same Operator Group and Calling Group, and the ring mode is set to Ring All. When an internal call is made to any extension the phones light up and each one rings. However, when a call comes in from an outside extension, only the first two extensions (the 1416 and the first 1408) light up and ring correctly. The other three light up and the Caller ID starts, but they don't ring. I've checked to make sure the Audible Alerts are set correctly, there's not a privacy button or DND button that's pressed, and nothing has changed in the configuration as far as I can tell. I'm told the phones were working correctly up until yesterday. This is also affecting the two cordless phones on the system as well - they light up and Caller ID works, but there's no ring. Any ideas on what could be the issue? The site is on a UPS and everything is working normally except for the ringing issue on the (4) 1408's and (2) cordless sets.