If the IP Phone has never been entered into the CallManager database via CallManager Administration, CallManager does not allow that phone to register unless auto-registration is enabled. Auto-registration does not work unless it has available directory numbers to provide the registering IP Phone a DN. If the IP Phone is not configured in CallManager Administration and auto-registration is not enabled or CallManager has run out of DNs, the phone displays "Registration Rejected."
In some cases, the phone might be having old residual security config that is no longer needed, in such case you need to factory reset.
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