Can an IP phone be configured so that calls to that IP phone can be rerouted if the IP phone
become unregistered?

A.
Yes, configuring call forward All provides this coverage

B.
No, if a phone is unregistered, the re-order tone is played to the caller

C.
Yes, configuring Call Forward Busy Internal provides this coverage

D.
Yes, configuring Call Forward Unregistered provides this coverage

E.
No. the call is dropped at the gateway